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* [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12)
@ 2020-01-29  3:24 John Hubbard
  2020-01-29  3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
                   ` (7 more replies)
  0 siblings, 8 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

OK, as requested, I've split the tracking patch into 6 smaller patches,
and it should be *much* easier to understand and review now.

============================================================
Changes since v1:

* Split the tracking patch into 6 smaller patches

* Rebased onto today's linux-next/akpm (there weren't any conflicts).

* Fixed an "unsigned int" vs. "int" problem in gup_benchmark, reported
  by Nathan Chancellor. (I don't see it in my local builds, probably
  because they use gcc, but an LLVM test found the mismatch.)

* Fixed a huge page pincount problem (add/subtract vs.
  increment/decrement), spotted by Jan Kara.
============================================================

There is a reasonable case to be made for merging two of the patches
(patches 4 and 5), given that patch 4 provides tracking that has upper
limits on the number of pins that can be done with huge pages. Let me
know if anyone wants those merged, but unless there is some weird chance
of someone grabbing patch 4 and not patch 5, I don't really see the
need. Meanwhile, it's easier to review in this form.

Also, patch 3 has been revived. Earlier reviewers asked for it to be
merged into the tracking patch (one cannot please everyone, heh), but
now it's back out on it's own.

This activates tracking of FOLL_PIN pages. This is in support of fixing
the get_user_pages()+DMA problem described in [1]-[4].

It is based on today's (Jan 28) linux-next (branch: akpm),
commit 280e9cb00b41 ("drivers/media/platform/sti/delta/delta-ipc.c: fix
read buffer overflow")

There is a git repo and branch, for convenience in reviewing:

    git@github.com:johnhubbard/linux.git
            track_user_pages_v2_linux-next_akpm_28Jan2020

FOLL_PIN support is (so far) in mmotm and linux-next. However, the
patch to use FOLL_PIN to track pages was *not* submitted, because Leon
saw an RDMA test suite failure that involved (I think) page refcount
overflows when huge pages were used.

This patch definitively solves that kind of overflow problem, by adding
an exact pincount, for compound pages (of order > 1), in the 3rd struct
page of a compound page. If available, that form of pincounting is used,
instead of the GUP_PIN_COUNTING_BIAS approach. Thanks again to Jan Kara
for that idea.

Here's the last reviewed version of the tracking patch (v11):

  https://lore.kernel.org/r/20191216222537.491123-1-jhubbard@nvidia.com

Jan Kara had provided a reviewed-by tag for that, but I've had to remove
it (again) here, due to having changed the patch "a little bit", in
order to add the feature described above.

Other interesting changes:

* dump_page(): added one, or two new things to report for compound
  pages: head refcount (for all compound pages), and map_pincount (for
  compound pages of order > 1).

* Documentation/core-api/pin_user_pages.rst: removed the "TODO" for the
  huge page refcount upper limit problems, and added notes about how it
  works now. Also added a note about the dump_page() enhancements.

* Added some comments in gup.c and mm.h, to explain that there are two
  ways to count pinned pages: exact (for compound pages of order > 1)
  and fuzzy (GUP_PIN_COUNTING_BIAS: for all other pages).

============================================================
General notes about the tracking patch:

This is a prerequisite to solving the problem of proper interactions
between file-backed pages, and [R]DMA activities, as discussed in [1],
[2], [3], [4] and in a remarkable number of email threads since about
2017. :)

In contrast to earlier approaches, the page tracking can be
incrementally applied to the kernel call sites that, until now, have
been simply calling get_user_pages() ("gup"). In other words, opt-in by
changing from this:

    get_user_pages() (sets FOLL_GET)
    put_page()

to this:
    pin_user_pages() (sets FOLL_PIN)
    unpin_user_page()

============================================================
Next steps:

* Convert more subsystems from get_user_pages() to pin_user_pages().
* Work with Ira and others to connect this all up with file system
  leases.

[1] Some slow progress on get_user_pages() (Apr 2, 2019):
    https://lwn.net/Articles/784574/

[2] DMA and get_user_pages() (LPC: Dec 12, 2018):
    https://lwn.net/Articles/774411/

[3] The trouble with get_user_pages() (Apr 30, 2018):
    https://lwn.net/Articles/753027/

[4] LWN kernel index: get_user_pages()
    https://lwn.net/Kernel/Index/#Memory_management-get_user_pages


John Hubbard (8):
  mm: dump_page: print head page's refcount, for compound pages
  mm/gup: split get_user_pages_remote() into two routines
  mm/gup: pass a flags arg to __gup_device_* functions
  mm/gup: track FOLL_PIN pages
  mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages
  mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
  mm/gup_benchmark: support pin_user_pages() and related calls
  selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN
    coverage

 Documentation/core-api/pin_user_pages.rst  |  47 +--
 include/linux/mm.h                         | 109 ++++-
 include/linux/mm_types.h                   |   7 +-
 include/linux/mmzone.h                     |   2 +
 include/linux/page_ref.h                   |  10 +
 mm/debug.c                                 |  22 +-
 mm/gup.c                                   | 460 ++++++++++++++++-----
 mm/gup_benchmark.c                         |  71 +++-
 mm/huge_memory.c                           |  29 +-
 mm/hugetlb.c                               |  44 +-
 mm/page_alloc.c                            |   2 +
 mm/rmap.c                                  |   6 +
 mm/vmstat.c                                |   2 +
 tools/testing/selftests/vm/gup_benchmark.c |  15 +-
 tools/testing/selftests/vm/run_vmtests     |  22 +
 15 files changed, 681 insertions(+), 167 deletions(-)

-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29 11:25   ` Kirill A. Shutemov
  2020-01-29  3:24 ` [PATCH v2 2/8] mm/gup: split get_user_pages_remote() into two routines John Hubbard
                   ` (6 subsequent siblings)
  7 siblings, 1 reply; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

When debugging a problem that involves compound pages, it is extremely
helpful if dump_page() reports not only the page->_refcount, but also
the refcount of the head page of the compound page. That's because the
head page collects refcounts for the entire compound page.

Therefore, enhance dump_page() so as to print out the refcount of the
head page of a compound page.

This approach (printing information about a struct page that is not the
struct page that was passed into dump_page()) has a precedent:
compound_mapcount is already being printed.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/debug.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..4cc6cad8385d 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -76,9 +76,11 @@ void __dump_page(struct page *page, const char *reason)
 	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
 
 	if (PageCompound(page))
-		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
-			"index:%#lx compound_mapcount: %d\n",
-			page, page_ref_count(page), mapcount,
+		pr_warn("page:%px refcount:%d head refcount:%d "
+			"mapcount:%d mapping:%px index:%#lx "
+			"compound_mapcount:%d\n",
+			page, page_ref_count(page),
+			page_ref_count(compound_head(page)), mapcount,
 			page->mapping, page_to_pgoff(page),
 			compound_mapcount(page));
 	else
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 2/8] mm/gup: split get_user_pages_remote() into two routines
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
  2020-01-29  3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29  3:24 ` [PATCH v2 3/8] mm/gup: pass a flags arg to __gup_device_* functions John Hubbard
                   ` (5 subsequent siblings)
  7 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

An upcoming patch requires reusing the implementation of
get_user_pages_remote(). Split up get_user_pages_remote() into an outer
routine that checks flags, and an implementation routine that will be
reused. This makes subsequent changes much easier to understand.

There should be no change in behavior due to this patch.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 56 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 33 insertions(+), 23 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 1b521e0ac1de..b699500da077 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1557,6 +1557,37 @@ static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
 }
 #endif /* CONFIG_FS_DAX || CONFIG_CMA */
 
+#ifdef CONFIG_MMU
+static long __get_user_pages_remote(struct task_struct *tsk,
+				    struct mm_struct *mm,
+				    unsigned long start, unsigned long nr_pages,
+				    unsigned int gup_flags, struct page **pages,
+				    struct vm_area_struct **vmas, int *locked)
+{
+	/*
+	 * Parts of FOLL_LONGTERM behavior are incompatible with
+	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
+	 * vmas. However, this only comes up if locked is set, and there are
+	 * callers that do request FOLL_LONGTERM, but do not set locked. So,
+	 * allow what we can.
+	 */
+	if (gup_flags & FOLL_LONGTERM) {
+		if (WARN_ON_ONCE(locked))
+			return -EINVAL;
+		/*
+		 * This will check the vmas (even if our vmas arg is NULL)
+		 * and return -ENOTSUPP if DAX isn't allowed in this case:
+		 */
+		return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
+					     vmas, gup_flags | FOLL_TOUCH |
+					     FOLL_REMOTE);
+	}
+
+	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
+				       locked,
+				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
+}
+
 /*
  * get_user_pages_remote() - pin user pages in memory
  * @tsk:	the task_struct to use for page fault accounting, or
@@ -1619,7 +1650,6 @@ static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
  * should use get_user_pages because it cannot pass
  * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
  */
-#ifdef CONFIG_MMU
 long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 		unsigned long start, unsigned long nr_pages,
 		unsigned int gup_flags, struct page **pages,
@@ -1632,28 +1662,8 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
 		return -EINVAL;
 
-	/*
-	 * Parts of FOLL_LONGTERM behavior are incompatible with
-	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
-	 * vmas. However, this only comes up if locked is set, and there are
-	 * callers that do request FOLL_LONGTERM, but do not set locked. So,
-	 * allow what we can.
-	 */
-	if (gup_flags & FOLL_LONGTERM) {
-		if (WARN_ON_ONCE(locked))
-			return -EINVAL;
-		/*
-		 * This will check the vmas (even if our vmas arg is NULL)
-		 * and return -ENOTSUPP if DAX isn't allowed in this case:
-		 */
-		return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
-					     vmas, gup_flags | FOLL_TOUCH |
-					     FOLL_REMOTE);
-	}
-
-	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
-				       locked,
-				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
+	return __get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags,
+				       pages, vmas, locked);
 }
 EXPORT_SYMBOL(get_user_pages_remote);
 
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 3/8] mm/gup: pass a flags arg to __gup_device_* functions
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
  2020-01-29  3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
  2020-01-29  3:24 ` [PATCH v2 2/8] mm/gup: split get_user_pages_remote() into two routines John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29  3:24 ` [PATCH v2 4/8] mm/gup: track FOLL_PIN pages John Hubbard
                   ` (4 subsequent siblings)
  7 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard, Kirill A . Shutemov

A subsequent patch requires access to gup flags, so pass the flags
argument through to the __gup_device_* functions.

Also placate checkpatch.pl by shortening a nearby line.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index b699500da077..9e117998274c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1963,7 +1963,8 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 
 #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
 static int __gup_device_huge(unsigned long pfn, unsigned long addr,
-		unsigned long end, struct page **pages, int *nr)
+			     unsigned long end, unsigned int flags,
+			     struct page **pages, int *nr)
 {
 	int nr_start = *nr;
 	struct dev_pagemap *pgmap = NULL;
@@ -1989,13 +1990,14 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
 }
 
 static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
-		unsigned long end, struct page **pages, int *nr)
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
 {
 	unsigned long fault_pfn;
 	int nr_start = *nr;
 
 	fault_pfn = pmd_pfn(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
-	if (!__gup_device_huge(fault_pfn, addr, end, pages, nr))
+	if (!__gup_device_huge(fault_pfn, addr, end, flags, pages, nr))
 		return 0;
 
 	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
@@ -2006,13 +2008,14 @@ static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 }
 
 static int __gup_device_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
-		unsigned long end, struct page **pages, int *nr)
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
 {
 	unsigned long fault_pfn;
 	int nr_start = *nr;
 
 	fault_pfn = pud_pfn(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
-	if (!__gup_device_huge(fault_pfn, addr, end, pages, nr))
+	if (!__gup_device_huge(fault_pfn, addr, end, flags, pages, nr))
 		return 0;
 
 	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
@@ -2023,14 +2026,16 @@ static int __gup_device_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 }
 #else
 static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
-		unsigned long end, struct page **pages, int *nr)
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
 {
 	BUILD_BUG();
 	return 0;
 }
 
 static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
-		unsigned long end, struct page **pages, int *nr)
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
 {
 	BUILD_BUG();
 	return 0;
@@ -2146,7 +2151,8 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	if (pmd_devmap(orig)) {
 		if (unlikely(flags & FOLL_LONGTERM))
 			return 0;
-		return __gup_device_huge_pmd(orig, pmdp, addr, end, pages, nr);
+		return __gup_device_huge_pmd(orig, pmdp, addr, end, flags,
+					     pages, nr);
 	}
 
 	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
@@ -2167,7 +2173,8 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 }
 
 static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
-		unsigned long end, unsigned int flags, struct page **pages, int *nr)
+			unsigned long end, unsigned int flags,
+			struct page **pages, int *nr)
 {
 	struct page *head, *page;
 	int refs;
@@ -2178,7 +2185,8 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	if (pud_devmap(orig)) {
 		if (unlikely(flags & FOLL_LONGTERM))
 			return 0;
-		return __gup_device_huge_pud(orig, pudp, addr, end, pages, nr);
+		return __gup_device_huge_pud(orig, pudp, addr, end, flags,
+					     pages, nr);
 	}
 
 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 4/8] mm/gup: track FOLL_PIN pages
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
                   ` (2 preceding siblings ...)
  2020-01-29  3:24 ` [PATCH v2 3/8] mm/gup: pass a flags arg to __gup_device_* functions John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29 13:51   ` Kirill A. Shutemov
  2020-01-29  3:24 ` [PATCH v2 5/8] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages John Hubbard
                   ` (3 subsequent siblings)
  7 siblings, 1 reply; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard, Kirill A . Shutemov

Add tracking of pages that were pinned via FOLL_PIN. This tracking is
implemented via overloading of page->_refcount: pins are added by
adding GUP_PIN_COUNTING_BIAS (1024) to the refcount. This provides a
fuzzy indication of pinning, and it can have false positives (and that's
OK). Please see the pre-existing
Documentation/core-api/pin_user_pages.rst for details.

As mentioned in pin_user_pages.rst, callers who effectively set FOLL_PIN
(typically via pin_user_pages*()) are required to ultimately free such
pages via unpin_user_page().

Please also not the limitation, discussed in pin_user_pages.rst under
the "TODO: for 1GB and larger huge pages" section. (That limitation will
be removed in a following patch.)

The effect of a FOLL_PIN flag is similar to that of FOLL_GET, and may be
thought of as "FOLL_GET for DIO and/or RDMA use".

Pages that have been pinned via FOLL_PIN are identifiable via a
new function call:

   bool page_dma_pinned(struct page *page);

What to do in response to encountering such a page, is left to later
patchsets. There is discussion about this in [1], [2], [3], and [4].

This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().

[1] Some slow progress on get_user_pages() (Apr 2, 2019):
    https://lwn.net/Articles/784574/
[2] DMA and get_user_pages() (LPC: Dec 12, 2018):
    https://lwn.net/Articles/774411/
[3] The trouble with get_user_pages() (Apr 30, 2018):
    https://lwn.net/Articles/753027/
[4] LWN kernel index: get_user_pages():
    https://lwn.net/Kernel/Index/#Memory_management-get_user_pages

Suggested-by: Jan Kara <jack@suse.cz>
Suggested-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h       |  83 +++++++++--
 include/linux/page_ref.h |  10 ++
 mm/gup.c                 | 293 +++++++++++++++++++++++++++++++--------
 mm/huge_memory.c         |  29 +++-
 mm/hugetlb.c             |  38 ++---
 5 files changed, 354 insertions(+), 99 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 080f8ac8bfb7..c5d0f4a66788 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1001,6 +1001,8 @@ static inline void get_page(struct page *page)
 	page_ref_inc(page);
 }
 
+bool __must_check try_grab_page(struct page *page, unsigned int flags);
+
 static inline __must_check bool try_get_page(struct page *page)
 {
 	page = compound_head(page);
@@ -1029,29 +1031,80 @@ static inline void put_page(struct page *page)
 		__put_page(page);
 }
 
-/**
- * unpin_user_page() - release a gup-pinned page
- * @page:            pointer to page to be released
+/*
+ * GUP_PIN_COUNTING_BIAS, and the associated functions that use it, overload
+ * the page's refcount so that two separate items are tracked: the original page
+ * reference count, and also a new count of how many pin_user_pages() calls were
+ * made against the page. ("gup-pinned" is another term for the latter).
+ *
+ * With this scheme, pin_user_pages() becomes special: such pages are marked as
+ * distinct from normal pages. As such, the unpin_user_page() call (and its
+ * variants) must be used in order to release gup-pinned pages.
+ *
+ * Choice of value:
+ *
+ * By making GUP_PIN_COUNTING_BIAS a power of two, debugging of page reference
+ * counts with respect to pin_user_pages() and unpin_user_page() becomes
+ * simpler, due to the fact that adding an even power of two to the page
+ * refcount has the effect of using only the upper N bits, for the code that
+ * counts up using the bias value. This means that the lower bits are left for
+ * the exclusive use of the original code that increments and decrements by one
+ * (or at least, by much smaller values than the bias value).
  *
- * Pages that were pinned via pin_user_pages*() must be released via either
- * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
- * that eventually such pages can be separately tracked and uniquely handled. In
- * particular, interactions with RDMA and filesystems need special handling.
+ * Of course, once the lower bits overflow into the upper bits (and this is
+ * OK, because subtraction recovers the original values), then visual inspection
+ * no longer suffices to directly view the separate counts. However, for normal
+ * applications that don't have huge page reference counts, this won't be an
+ * issue.
  *
- * unpin_user_page() and put_page() are not interchangeable, despite this early
- * implementation that makes them look the same. unpin_user_page() calls must
- * be perfectly matched up with pin*() calls.
+ * Locking: the lockless algorithm described in page_cache_get_speculative()
+ * and page_cache_gup_pin_speculative() provides safe operation for
+ * get_user_pages and page_mkclean and other calls that race to set up page
+ * table entries.
  */
-static inline void unpin_user_page(struct page *page)
-{
-	put_page(page);
-}
+#define GUP_PIN_COUNTING_BIAS (1U << 10)
 
+void unpin_user_page(struct page *page);
 void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 				 bool make_dirty);
-
 void unpin_user_pages(struct page **pages, unsigned long npages);
 
+/**
+ * page_dma_pinned() - report if a page is pinned for DMA.
+ *
+ * This function checks if a page has been pinned via a call to
+ * pin_user_pages*().
+ *
+ * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
+ * because it means "definitely not pinned for DMA", but true means "probably
+ * pinned for DMA, but possibly a false positive due to having at least
+ * GUP_PIN_COUNTING_BIAS worth of normal page references".
+ *
+ * False positives are OK, because: a) it's unlikely for a page to get that many
+ * refcounts, and b) all the callers of this routine are expected to be able to
+ * deal gracefully with a false positive.
+ *
+ * For more information, please see Documentation/vm/pin_user_pages.rst.
+ *
+ * @page:	pointer to page to be queried.
+ * @Return:	True, if it is likely that the page has been "dma-pinned".
+ *		False, if the page is definitely not dma-pinned.
+ */
+static inline bool page_dma_pinned(struct page *page)
+{
+	/*
+	 * page_ref_count() is signed. If that refcount overflows, then
+	 * page_ref_count() returns a negative value, and callers will avoid
+	 * further incrementing the refcount.
+	 *
+	 * Here, for that overflow case, use the signed bit to count a little
+	 * bit higher via unsigned math, and thus still get an accurate result
+	 * from page_dma_pinned().
+	 */
+	return ((unsigned int)page_ref_count(compound_head(page))) >=
+		GUP_PIN_COUNTING_BIAS;
+}
+
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define SECTION_IN_PAGE_FLAGS
 #endif
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 14d14beb1f7f..b9cbe553d1e7 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -102,6 +102,16 @@ static inline void page_ref_sub(struct page *page, int nr)
 		__page_ref_mod(page, -nr);
 }
 
+static inline int page_ref_sub_return(struct page *page, int nr)
+{
+	int ret = atomic_sub_return(nr, &page->_refcount);
+
+	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
+		__page_ref_mod(page, -nr);
+
+	return ret;
+}
+
 static inline void page_ref_inc(struct page *page)
 {
 	atomic_inc(&page->_refcount);
diff --git a/mm/gup.c b/mm/gup.c
index 9e117998274c..7a96490dcc54 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -44,6 +44,136 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
 	return head;
 }
 
+/*
+ * try_grab_compound_head() - attempt to elevate a page's refcount, by a
+ * flags-dependent amount.
+ *
+ * "grab" names in this file mean, "look at flags to decide whether to use
+ * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
+ *
+ * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
+ * same time. (That's true throughout the get_user_pages*() and
+ * pin_user_pages*() APIs.) Cases:
+ *
+ *    FOLL_GET: page's refcount will be incremented by 1.
+ *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *
+ * Return: head page (with refcount appropriately incremented) for success, or
+ * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
+ * considered failure, and furthermore, a likely bug in the caller, so a warning
+ * is also emitted.
+ */
+static __maybe_unused struct page *try_grab_compound_head(struct page *page,
+							  int refs,
+							  unsigned int flags)
+{
+	if (flags & FOLL_GET)
+		return try_get_compound_head(page, refs);
+	else if (flags & FOLL_PIN) {
+		refs *= GUP_PIN_COUNTING_BIAS;
+		return try_get_compound_head(page, refs);
+	}
+
+	WARN_ON_ONCE(1);
+	return NULL;
+}
+
+/**
+ * try_grab_page() - elevate a page's refcount by a flag-dependent amount
+ *
+ * This might not do anything at all, depending on the flags argument.
+ *
+ * "grab" names in this file mean, "look at flags to decide whether to use
+ * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
+ *
+ * @page:    pointer to page to be grabbed
+ * @flags:   gup flags: these are the FOLL_* flag values.
+ *
+ * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
+ * time. Cases:
+ *
+ *    FOLL_GET: page's refcount will be incremented by 1.
+ *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *
+ * Return: true for success, or if no action was required (if neither FOLL_PIN
+ * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
+ * FOLL_PIN was set, but the page could not be grabbed.
+ */
+bool __must_check try_grab_page(struct page *page, unsigned int flags)
+{
+	if (flags & FOLL_GET)
+		return try_get_page(page);
+	else if (flags & FOLL_PIN) {
+		page = compound_head(page);
+
+		if (WARN_ON_ONCE(flags & FOLL_GET))
+			return false;
+
+		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
+			return false;
+
+		page_ref_add(page, GUP_PIN_COUNTING_BIAS);
+	}
+
+	return true;
+}
+
+#ifdef CONFIG_DEV_PAGEMAP_OPS
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+	int count;
+
+	if (!page_is_devmap_managed(page))
+		return false;
+
+	count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+
+	/*
+	 * devmap page refcounts are 1-based, rather than 0-based: if
+	 * refcount is 1, then the page is free and the refcount is
+	 * stable because nobody holds a reference on the page.
+	 */
+	if (count == 1)
+		free_devmap_managed_page(page);
+	else if (!count)
+		__put_page(page);
+
+	return true;
+}
+#else
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+	return false;
+}
+#endif /* CONFIG_DEV_PAGEMAP_OPS */
+
+/**
+ * unpin_user_page() - release a dma-pinned page
+ * @page:            pointer to page to be released
+ *
+ * Pages that were pinned via pin_user_pages*() must be released via either
+ * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
+ * that such pages can be separately tracked and uniquely handled. In
+ * particular, interactions with RDMA and filesystems need special handling.
+ */
+void unpin_user_page(struct page *page)
+{
+	page = compound_head(page);
+
+	/*
+	 * For devmap managed pages we need to catch refcount transition from
+	 * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
+	 * page is free and we need to inform the device driver through
+	 * callback. See include/linux/memremap.h and HMM for details.
+	 */
+	if (__unpin_devmap_managed_user_page(page))
+		return;
+
+	if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+		__put_page(page);
+}
+EXPORT_SYMBOL(unpin_user_page);
+
 /**
  * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
  * @pages:  array of pages to be maybe marked dirty, and definitely released.
@@ -230,10 +360,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 	}
 
 	page = vm_normal_page(vma, address, pte);
-	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
+	if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
 		/*
-		 * Only return device mapping pages in the FOLL_GET case since
-		 * they are only valid while holding the pgmap reference.
+		 * Only return device mapping pages in the FOLL_GET or FOLL_PIN
+		 * case since they are only valid while holding the pgmap
+		 * reference.
 		 */
 		*pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);
 		if (*pgmap)
@@ -271,11 +402,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 		goto retry;
 	}
 
-	if (flags & FOLL_GET) {
-		if (unlikely(!try_get_page(page))) {
-			page = ERR_PTR(-ENOMEM);
-			goto out;
-		}
+	/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
+	if (unlikely(!try_grab_page(page, flags))) {
+		page = ERR_PTR(-ENOMEM);
+		goto out;
 	}
 	if (flags & FOLL_TOUCH) {
 		if ((flags & FOLL_WRITE) &&
@@ -537,7 +667,7 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 	/* make this handle hugepd */
 	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
 	if (!IS_ERR(page)) {
-		BUG_ON(flags & FOLL_GET);
+		WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
 		return page;
 	}
 
@@ -1675,6 +1805,15 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 {
 	return 0;
 }
+
+static long __get_user_pages_remote(struct task_struct *tsk,
+				    struct mm_struct *mm,
+				    unsigned long start, unsigned long nr_pages,
+				    unsigned int gup_flags, struct page **pages,
+				    struct vm_area_struct **vmas, int *locked)
+{
+	return 0;
+}
 #endif /* !CONFIG_MMU */
 
 /*
@@ -1870,13 +2009,17 @@ static inline pte_t gup_get_pte(pte_t *ptep)
 #endif /* CONFIG_GUP_GET_PTE_LOW_HIGH */
 
 static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
+					    unsigned int flags,
 					    struct page **pages)
 {
 	while ((*nr) - nr_start) {
 		struct page *page = pages[--(*nr)];
 
 		ClearPageReferenced(page);
-		put_page(page);
+		if (flags & FOLL_PIN)
+			unpin_user_page(page);
+		else
+			put_page(page);
 	}
 }
 
@@ -1909,7 +2052,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 
 			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
 			if (unlikely(!pgmap)) {
-				undo_dev_pagemap(nr, nr_start, pages);
+				undo_dev_pagemap(nr, nr_start, flags, pages);
 				goto pte_unmap;
 			}
 		} else if (pte_special(pte))
@@ -1918,7 +2061,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
 
-		head = try_get_compound_head(page, 1);
+		head = try_grab_compound_head(page, 1, flags);
 		if (!head)
 			goto pte_unmap;
 
@@ -1974,12 +2117,15 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
 
 		pgmap = get_dev_pagemap(pfn, pgmap);
 		if (unlikely(!pgmap)) {
-			undo_dev_pagemap(nr, nr_start, pages);
+			undo_dev_pagemap(nr, nr_start, flags, pages);
 			return 0;
 		}
 		SetPageReferenced(page);
 		pages[*nr] = page;
-		get_page(page);
+		if (unlikely(!try_grab_page(page, flags))) {
+			undo_dev_pagemap(nr, nr_start, flags, pages);
+			return 0;
+		}
 		(*nr)++;
 		pfn++;
 	} while (addr += PAGE_SIZE, addr != end);
@@ -2001,7 +2147,7 @@ static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 		return 0;
 
 	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
-		undo_dev_pagemap(nr, nr_start, pages);
+		undo_dev_pagemap(nr, nr_start, flags, pages);
 		return 0;
 	}
 	return 1;
@@ -2019,7 +2165,7 @@ static int __gup_device_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 		return 0;
 
 	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
-		undo_dev_pagemap(nr, nr_start, pages);
+		undo_dev_pagemap(nr, nr_start, flags, pages);
 		return 0;
 	}
 	return 1;
@@ -2053,8 +2199,11 @@ static int record_subpages(struct page *page, unsigned long addr,
 	return nr;
 }
 
-static void put_compound_head(struct page *page, int refs)
+static void put_compound_head(struct page *page, int refs, unsigned int flags)
 {
+	if (flags & FOLL_PIN)
+		refs *= GUP_PIN_COUNTING_BIAS;
+
 	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
 	/*
 	 * Calling put_page() for each ref is unnecessarily slow. Only the last
@@ -2098,12 +2247,12 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(head, refs);
+	head = try_grab_compound_head(head, refs, flags);
 	if (!head)
 		return 0;
 
 	if (unlikely(pte_val(pte) != pte_val(*ptep))) {
-		put_compound_head(head, refs);
+		put_compound_head(head, refs, flags);
 		return 0;
 	}
 
@@ -2158,12 +2307,12 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(pmd_page(orig), refs);
+	head = try_grab_compound_head(pmd_page(orig), refs, flags);
 	if (!head)
 		return 0;
 
 	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
-		put_compound_head(head, refs);
+		put_compound_head(head, refs, flags);
 		return 0;
 	}
 
@@ -2192,12 +2341,12 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(pud_page(orig), refs);
+	head = try_grab_compound_head(pud_page(orig), refs, flags);
 	if (!head)
 		return 0;
 
 	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
-		put_compound_head(head, refs);
+		put_compound_head(head, refs, flags);
 		return 0;
 	}
 
@@ -2221,12 +2370,12 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 	page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(pgd_page(orig), refs);
+	head = try_grab_compound_head(pgd_page(orig), refs, flags);
 	if (!head)
 		return 0;
 
 	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
-		put_compound_head(head, refs);
+		put_compound_head(head, refs, flags);
 		return 0;
 	}
 
@@ -2389,6 +2538,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	unsigned long len, end;
 	unsigned long flags;
 	int nr = 0;
+	/*
+	 * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
+	 * because gup fast is always a "pin with a +1 page refcount" request.
+	 */
+	unsigned int gup_flags = FOLL_GET;
+
+	if (write)
+		gup_flags |= FOLL_WRITE;
 
 	start = untagged_addr(start) & PAGE_MASK;
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
@@ -2414,7 +2571,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
 		local_irq_save(flags);
-		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
+		gup_pgd_range(start, end, gup_flags, pages, &nr);
 		local_irq_restore(flags);
 	}
 
@@ -2453,7 +2610,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 	int nr = 0, ret = 0;
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
-				       FOLL_FORCE | FOLL_PIN)))
+				       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
 		return -EINVAL;
 
 	start = untagged_addr(start) & PAGE_MASK;
@@ -2496,11 +2653,11 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 
 /**
  * get_user_pages_fast() - pin user pages in memory
- * @start:	starting user address
- * @nr_pages:	number of pages from start to pin
- * @gup_flags:	flags modifying pin behaviour
- * @pages:	array that receives pointers to the pages pinned.
- *		Should be at least nr_pages long.
+ * @start:      starting user address
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying pin behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long.
  *
  * Attempt to pin user pages in memory without taking mm->mmap_sem.
  * If not successful, it will fall back to taking the lock and
@@ -2520,6 +2677,13 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
 		return -EINVAL;
 
+	/*
+	 * The caller may or may not have explicitly set FOLL_GET; either way is
+	 * OK. However, internally (within mm/gup.c), gup fast variants must set
+	 * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
+	 * request.
+	 */
+	gup_flags |= FOLL_GET;
 	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(get_user_pages_fast);
@@ -2527,9 +2691,12 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
 /**
  * pin_user_pages_fast() - pin user pages in memory without taking locks
  *
- * For now, this is a placeholder function, until various call sites are
- * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
- * this is identical to get_user_pages_fast().
+ * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
+ * get_user_pages_fast() for documentation on the function arguments, because
+ * the arguments here are identical.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/vm/pin_user_pages.rst for further details.
  *
  * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
  * is NOT intended for Case 2 (RDMA: long-term pins).
@@ -2537,21 +2704,24 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
 int pin_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages)
 {
-	/*
-	 * This is a placeholder, until the pin functionality is activated.
-	 * Until then, just behave like the corresponding get_user_pages*()
-	 * routine.
-	 */
-	return get_user_pages_fast(start, nr_pages, gup_flags, pages);
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
 }
 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
 
 /**
  * pin_user_pages_remote() - pin pages of a remote process (task != current)
  *
- * For now, this is a placeholder function, until various call sites are
- * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
- * this is identical to get_user_pages_remote().
+ * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
+ * get_user_pages_remote() for documentation on the function arguments, because
+ * the arguments here are identical.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/vm/pin_user_pages.rst for details.
  *
  * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
  * is NOT intended for Case 2 (RDMA: long-term pins).
@@ -2561,22 +2731,24 @@ long pin_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 			   unsigned int gup_flags, struct page **pages,
 			   struct vm_area_struct **vmas, int *locked)
 {
-	/*
-	 * This is a placeholder, until the pin functionality is activated.
-	 * Until then, just behave like the corresponding get_user_pages*()
-	 * routine.
-	 */
-	return get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags, pages,
-				     vmas, locked);
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return __get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags,
+				       pages, vmas, locked);
 }
 EXPORT_SYMBOL(pin_user_pages_remote);
 
 /**
  * pin_user_pages() - pin user pages in memory for use by other devices
  *
- * For now, this is a placeholder function, until various call sites are
- * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
- * this is identical to get_user_pages().
+ * Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
+ * FOLL_PIN is set.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/vm/pin_user_pages.rst for details.
  *
  * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
  * is NOT intended for Case 2 (RDMA: long-term pins).
@@ -2585,11 +2757,12 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
 		    unsigned int gup_flags, struct page **pages,
 		    struct vm_area_struct **vmas)
 {
-	/*
-	 * This is a placeholder, until the pin functionality is activated.
-	 * Until then, just behave like the corresponding get_user_pages*()
-	 * routine.
-	 */
-	return get_user_pages(start, nr_pages, gup_flags, pages, vmas);
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return __gup_longterm_locked(current, current->mm, start, nr_pages,
+				     pages, vmas, gup_flags);
 }
 EXPORT_SYMBOL(pin_user_pages);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0a55dec68925..b1079aaa6f24 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 	 */
 	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
 
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return NULL;
+
 	if (flags & FOLL_WRITE && !pmd_write(*pmd))
 		return NULL;
 
@@ -973,7 +978,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 	 * device mapped pages can only be returned if the
 	 * caller will manage the page reference count.
 	 */
-	if (!(flags & FOLL_GET))
+	if (!(flags & (FOLL_GET | FOLL_PIN)))
 		return ERR_PTR(-EEXIST);
 
 	pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
@@ -981,7 +986,8 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 	if (!*pgmap)
 		return ERR_PTR(-EFAULT);
 	page = pfn_to_page(pfn);
-	get_page(page);
+	if (!try_grab_page(page, flags))
+		page = ERR_PTR(-ENOMEM);
 
 	return page;
 }
@@ -1101,6 +1107,11 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 	if (flags & FOLL_WRITE && !pud_write(*pud))
 		return NULL;
 
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return NULL;
+
 	if (pud_present(*pud) && pud_devmap(*pud))
 		/* pass */;
 	else
@@ -1112,8 +1123,10 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 	/*
 	 * device mapped pages can only be returned if the
 	 * caller will manage the page reference count.
+	 *
+	 * At least one of FOLL_GET | FOLL_PIN must be set, so assert that here:
 	 */
-	if (!(flags & FOLL_GET))
+	if (!(flags & (FOLL_GET | FOLL_PIN)))
 		return ERR_PTR(-EEXIST);
 
 	pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
@@ -1121,7 +1134,8 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 	if (!*pgmap)
 		return ERR_PTR(-EFAULT);
 	page = pfn_to_page(pfn);
-	get_page(page);
+	if (!try_grab_page(page, flags))
+		page = ERR_PTR(-ENOMEM);
 
 	return page;
 }
@@ -1497,8 +1511,13 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	page = pmd_page(*pmd);
 	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+
+	if (!try_grab_page(page, flags))
+		return ERR_PTR(-ENOMEM);
+
 	if (flags & FOLL_TOUCH)
 		touch_pmd(vma, addr, pmd, flags);
+
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
 		/*
 		 * We don't mlock() pte-mapped THPs. This way we can avoid
@@ -1535,8 +1554,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 skip_mlock:
 	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
 	VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
-	if (flags & FOLL_GET)
-		get_page(page);
 
 out:
 	return page;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dd8737a94bec..487e998fd38e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4375,19 +4375,6 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
 		page = pte_page(huge_ptep_get(pte));
 
-		/*
-		 * Instead of doing 'try_get_page()' below in the same_page
-		 * loop, just check the count once here.
-		 */
-		if (unlikely(page_count(page) <= 0)) {
-			if (pages) {
-				spin_unlock(ptl);
-				remainder = 0;
-				err = -ENOMEM;
-				break;
-			}
-		}
-
 		/*
 		 * If subpage information not requested, update counters
 		 * and skip the same_page loop below.
@@ -4405,7 +4392,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 same_page:
 		if (pages) {
 			pages[i] = mem_map_offset(page, pfn_offset);
-			get_page(pages[i]);
+			if (!try_grab_page(pages[i], flags)) {
+				spin_unlock(ptl);
+				remainder = 0;
+				err = -ENOMEM;
+				WARN_ON_ONCE(1);
+				break;
+			}
 		}
 
 		if (vmas)
@@ -4965,6 +4958,12 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 	struct page *page = NULL;
 	spinlock_t *ptl;
 	pte_t pte;
+
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return NULL;
+
 retry:
 	ptl = pmd_lockptr(mm, pmd);
 	spin_lock(ptl);
@@ -4977,8 +4976,11 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 	pte = huge_ptep_get((pte_t *)pmd);
 	if (pte_present(pte)) {
 		page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
-		if (flags & FOLL_GET)
-			get_page(page);
+		if (unlikely(!try_grab_page(page, flags))) {
+			WARN_ON_ONCE(1);
+			page = NULL;
+			goto out;
+		}
 	} else {
 		if (is_hugetlb_entry_migration(pte)) {
 			spin_unlock(ptl);
@@ -4999,7 +5001,7 @@ struct page * __weak
 follow_huge_pud(struct mm_struct *mm, unsigned long address,
 		pud_t *pud, int flags)
 {
-	if (flags & FOLL_GET)
+	if (flags & (FOLL_GET | FOLL_PIN))
 		return NULL;
 
 	return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
@@ -5008,7 +5010,7 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address,
 struct page * __weak
 follow_huge_pgd(struct mm_struct *mm, unsigned long address, pgd_t *pgd, int flags)
 {
-	if (flags & FOLL_GET)
+	if (flags & (FOLL_GET | FOLL_PIN))
 		return NULL;
 
 	return pte_page(*(pte_t *)pgd) + ((address & ~PGDIR_MASK) >> PAGE_SHIFT);
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 5/8] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
                   ` (3 preceding siblings ...)
  2020-01-29  3:24 ` [PATCH v2 4/8] mm/gup: track FOLL_PIN pages John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29  3:24 ` [PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting John Hubbard
                   ` (2 subsequent siblings)
  7 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

For huge pages (and in fact, any compound page), the
GUP_PIN_COUNTING_BIAS scheme tends to overflow too easily, each tail
page increments the head page->_refcount by GUP_PIN_COUNTING_BIAS
(1024). That limits the number of huge pages that can be pinned.

This patch removes that limitation, by using an exact form of pin
counting for compound pages of order > 1. The "order > 1" is required
because this approach uses the 3rd struct page in the compound page, and
order 1 compound pages only have two pages, so that won't work there.

A new struct page field, hpage_pinned_refcount, has been added,
replacing a padding field in the union (so no new space is used).

This enhancement also has a useful side effect: huge pages and compound
pages (of order > 1) do not suffer from the "potential false positives"
problem that is discussed in the page_dma_pinned() comment block. That
is because these compound pages have extra space for tracking things, so
they get exact pin counts instead of overloading page->_refcount.

dump_page() has also been enhanced slightly, to handle the new counting
field: for compound pages with order > 1, the exact pincount is
reported: page->hpage_pinned_refcount.

Documentation/core-api/pin_user_pages.rst is updated accordingly.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 Documentation/core-api/pin_user_pages.rst | 47 +++++++-------
 include/linux/mm.h                        | 26 ++++++++
 include/linux/mm_types.h                  |  7 +-
 mm/debug.c                                | 24 +++++--
 mm/gup.c                                  | 78 ++++++++++++++++++++---
 mm/hugetlb.c                              |  6 ++
 mm/page_alloc.c                           |  2 +
 mm/rmap.c                                 |  6 ++
 8 files changed, 157 insertions(+), 39 deletions(-)

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 1d490155ecd7..55a30d260d39 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -52,8 +52,22 @@ Which flags are set by each wrapper
 
 For these pin_user_pages*() functions, FOLL_PIN is OR'd in with whatever gup
 flags the caller provides. The caller is required to pass in a non-null struct
-pages* array, and the function then pin pages by incrementing each by a special
-value. For now, that value is +1, just like get_user_pages*().::
+pages* array, and the function then pins pages by incrementing each by a special
+value: GUP_PIN_COUNTING_BIAS.
+
+For huge pages (and in fact, any compound page of more than 2 pages), the
+GUP_PIN_COUNTING_BIAS scheme is not used. Instead, an exact form of pin counting
+is achieved, by using the 3rd struct page in the compound page. A new struct
+page field, hpage_pinned_refcount, has been added in order to support this.
+
+This approach for compound pages avoids the counting upper limit problems that
+are discussed below. Those limitations would have been aggravated severely by
+huge pages, because each tail page adds a refcount to the head page. And in
+fact, testing revealed that, without a separate hpage_pinned_refcount field,
+page overflows were seen in some huge page stress tests.
+
+This also means that huge pages and compound pages (of order > 1) do not suffer
+from the false positives problem that is mentioned below.::
 
  Function
  --------
@@ -99,27 +113,6 @@ pages:
 This also leads to limitations: there are only 31-10==21 bits available for a
 counter that increments 10 bits at a time.
 
-TODO: for 1GB and larger huge pages, this is cutting it close. That's because
-when pin_user_pages() follows such pages, it increments the head page by "1"
-(where "1" used to mean "+1" for get_user_pages(), but now means "+1024" for
-pin_user_pages()) for each tail page. So if you have a 1GB huge page:
-
-* There are 256K (18 bits) worth of 4 KB tail pages.
-* There are 21 bits available to count up via GUP_PIN_COUNTING_BIAS (that is,
-  10 bits at a time)
-* There are 21 - 18 == 3 bits available to count. Except that there aren't,
-  because you need to allow for a few normal get_page() calls on the head page,
-  as well. Fortunately, the approach of using addition, rather than "hard"
-  bitfields, within page->_refcount, allows for sharing these bits gracefully.
-  But we're still looking at about 8 references.
-
-This, however, is a missing feature more than anything else, because it's easily
-solved by addressing an obvious inefficiency in the original get_user_pages()
-approach of retrieving pages: stop treating all the pages as if they were
-PAGE_SIZE. Retrieve huge pages as huge pages. The callers need to be aware of
-this, so some work is required. Once that's in place, this limitation mostly
-disappears from view, because there will be ample refcounting range available.
-
 * Callers must specifically request "dma-pinned tracking of pages". In other
   words, just calling get_user_pages() will not suffice; a new set of functions,
   pin_user_page() and related, must be used.
@@ -222,11 +215,19 @@ Those are both going to show zero, unless CONFIG_DEBUG_VM is set. This is
 because there is a noticeable performance drop in unpin_user_page(), when they
 are activated.
 
+Other diagnostics
+=================
+
+dump_page() has been enhanced slightly, to handle these new counting fields, and
+to better report on compound pages in general. Specifically, for compound pages
+with order > 1, the exact (hpage_pinned_refcount) pincount is reported.
+
 References
 ==========
 
 * `Some slow progress on get_user_pages() (Apr 2, 2019) <https://lwn.net/Articles/784574/>`_
 * `DMA and get_user_pages() (LPC: Dec 12, 2018) <https://lwn.net/Articles/774411/>`_
 * `The trouble with get_user_pages() (Apr 30, 2018) <https://lwn.net/Articles/753027/>`_
+* `LWN kernel index: get_user_pages() <https://lwn.net/Kernel/Index/#Memory_management-get_user_pages>`
 
 John Hubbard, October, 2019
diff --git a/include/linux/mm.h b/include/linux/mm.h
index c5d0f4a66788..a8ad5612bbcb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -770,6 +770,24 @@ static inline unsigned int compound_order(struct page *page)
 	return page[1].compound_order;
 }
 
+static inline bool hpage_pincount_available(struct page *page)
+{
+	/*
+	 * Can the page->hpage_pinned_refcount field be used? That field is in
+	 * the 3rd page of the compound page, so the smallest (2-page) compound
+	 * pages cannot support it.
+	 */
+	page = compound_head(page);
+	return PageCompound(page) && compound_order(page) > 1;
+}
+
+static inline int compound_pincount(struct page *page)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	page = compound_head(page);
+	return atomic_read(compound_pincount_ptr(page));
+}
+
 static inline void set_compound_order(struct page *page, unsigned int order)
 {
 	page[1].compound_order = order;
@@ -1084,6 +1102,11 @@ void unpin_user_pages(struct page **pages, unsigned long npages);
  * refcounts, and b) all the callers of this routine are expected to be able to
  * deal gracefully with a false positive.
  *
+ * For huge pages, the result will be exactly correct. That's because we have
+ * more tracking data available: the 3rd struct page in the compound page is
+ * used to track the pincount (instead using of the GUP_PIN_COUNTING_BIAS
+ * scheme).
+ *
  * For more information, please see Documentation/vm/pin_user_pages.rst.
  *
  * @page:	pointer to page to be queried.
@@ -1092,6 +1115,9 @@ void unpin_user_pages(struct page **pages, unsigned long npages);
  */
 static inline bool page_dma_pinned(struct page *page)
 {
+	if (hpage_pincount_available(page))
+		return compound_pincount(page) > 0;
+
 	/*
 	 * page_ref_count() is signed. If that refcount overflows, then
 	 * page_ref_count() returns a negative value, and callers will avoid
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 270aa8fd2800..d165275d8929 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -137,7 +137,7 @@ struct page {
 		};
 		struct {	/* Second tail page of compound page */
 			unsigned long _compound_pad_1;	/* compound_head */
-			unsigned long _compound_pad_2;
+			atomic_t hpage_pinned_refcount;
 			/* For both global and memcg */
 			struct list_head deferred_list;
 		};
@@ -226,6 +226,11 @@ static inline atomic_t *compound_mapcount_ptr(struct page *page)
 	return &page[1].compound_mapcount;
 }
 
+static inline atomic_t *compound_pincount_ptr(struct page *page)
+{
+	return &page[2].hpage_pinned_refcount;
+}
+
 /*
  * Used for sizing the vmemmap region on some architectures
  */
diff --git a/mm/debug.c b/mm/debug.c
index 4cc6cad8385d..8db7f36c7bc6 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -76,13 +76,23 @@ void __dump_page(struct page *page, const char *reason)
 	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
 
 	if (PageCompound(page))
-		pr_warn("page:%px refcount:%d head refcount:%d "
-			"mapcount:%d mapping:%px index:%#lx "
-			"compound_mapcount:%d\n",
-			page, page_ref_count(page),
-			page_ref_count(compound_head(page)), mapcount,
-			page->mapping, page_to_pgoff(page),
-			compound_mapcount(page));
+		if (hpage_pincount_available(page))
+			pr_warn("page:%px refcount:%d head refcount:%d "
+				"mapcount:%d mapping:%px index:%#lx "
+				"compound_mapcount:%d compound_pincount:%d\n",
+				page, page_ref_count(page),
+				page_ref_count(compound_head(page)), mapcount,
+				page->mapping, page_to_pgoff(page),
+				compound_mapcount(page),
+				compound_pincount(page));
+		else
+			pr_warn("page:%px refcount:%d head refcount:%d "
+				"mapcount:%d mapping:%px index:%#lx "
+				"compound_mapcount:%d\n",
+				page, page_ref_count(page),
+				page_ref_count(compound_head(page)), mapcount,
+				page->mapping, page_to_pgoff(page),
+				compound_mapcount(page));
 	else
 		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
 			page, page_ref_count(page), mapcount,
diff --git a/mm/gup.c b/mm/gup.c
index 7a96490dcc54..03e7a5cfa6a9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -29,6 +29,22 @@ struct follow_page_context {
 	unsigned int page_mask;
 };
 
+static void hpage_pincount_add(struct page *page, int refs)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+	atomic_add(refs, compound_pincount_ptr(page));
+}
+
+static void hpage_pincount_sub(struct page *page, int refs)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+	atomic_sub(refs, compound_pincount_ptr(page));
+}
+
 /*
  * Return the compound head page with ref appropriately incremented,
  * or NULL if that failed.
@@ -70,8 +86,25 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 	if (flags & FOLL_GET)
 		return try_get_compound_head(page, refs);
 	else if (flags & FOLL_PIN) {
-		refs *= GUP_PIN_COUNTING_BIAS;
-		return try_get_compound_head(page, refs);
+		/*
+		 * When pinning a compound page of order > 1 (which is what
+		 * hpage_pincount_available() checks for), use an exact count to
+		 * track it, via hpage_pincount_add/_sub().
+		 *
+		 * However, be sure to *also* increment the normal page refcount
+		 * field at least once, so that the page really is pinned.
+		 */
+		if (!hpage_pincount_available(page))
+			refs *= GUP_PIN_COUNTING_BIAS;
+
+		page = try_get_compound_head(page, refs);
+		if (!page)
+			return NULL;
+
+		if (hpage_pincount_available(page))
+			hpage_pincount_add(page, refs);
+
+		return page;
 	}
 
 	WARN_ON_ONCE(1);
@@ -104,6 +137,8 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 	if (flags & FOLL_GET)
 		return try_get_page(page);
 	else if (flags & FOLL_PIN) {
+		int refs = 1;
+
 		page = compound_head(page);
 
 		if (WARN_ON_ONCE(flags & FOLL_GET))
@@ -112,7 +147,18 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
 			return false;
 
-		page_ref_add(page, GUP_PIN_COUNTING_BIAS);
+		if (hpage_pincount_available(page))
+			hpage_pincount_add(page, 1);
+		else
+			refs = GUP_PIN_COUNTING_BIAS;
+
+		/*
+		 * Similar to try_grab_compound_head(): even if using the
+		 * hpage_pincount_add/_sub() routines, be sure to
+		 * *also* increment the normal page refcount field at least
+		 * once, so that the page really is pinned.
+		 */
+		page_ref_add(page, refs);
 	}
 
 	return true;
@@ -121,12 +167,17 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 #ifdef CONFIG_DEV_PAGEMAP_OPS
 static bool __unpin_devmap_managed_user_page(struct page *page)
 {
-	int count;
+	int count, refs = 1;
 
 	if (!page_is_devmap_managed(page))
 		return false;
 
-	count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+	if (hpage_pincount_available(page))
+		hpage_pincount_sub(page, 1);
+	else
+		refs = GUP_PIN_COUNTING_BIAS;
+
+	count = page_ref_sub_return(page, refs);
 
 	/*
 	 * devmap page refcounts are 1-based, rather than 0-based: if
@@ -158,6 +209,8 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
  */
 void unpin_user_page(struct page *page)
 {
+	int refs = 1;
+
 	page = compound_head(page);
 
 	/*
@@ -169,7 +222,12 @@ void unpin_user_page(struct page *page)
 	if (__unpin_devmap_managed_user_page(page))
 		return;
 
-	if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+	if (hpage_pincount_available(page))
+		hpage_pincount_sub(page, 1);
+	else
+		refs = GUP_PIN_COUNTING_BIAS;
+
+	if (page_ref_sub_and_test(page, refs))
 		__put_page(page);
 }
 EXPORT_SYMBOL(unpin_user_page);
@@ -2201,8 +2259,12 @@ static int record_subpages(struct page *page, unsigned long addr,
 
 static void put_compound_head(struct page *page, int refs, unsigned int flags)
 {
-	if (flags & FOLL_PIN)
-		refs *= GUP_PIN_COUNTING_BIAS;
+	if (flags & FOLL_PIN) {
+		if (hpage_pincount_available(page))
+			hpage_pincount_sub(page, refs);
+		else
+			refs *= GUP_PIN_COUNTING_BIAS;
+	}
 
 	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
 	/*
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 487e998fd38e..07059d936f7b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1009,6 +1009,9 @@ static void destroy_compound_gigantic_page(struct page *page,
 	struct page *p = page + 1;
 
 	atomic_set(compound_mapcount_ptr(page), 0);
+	if (hpage_pincount_available(page))
+		atomic_set(compound_pincount_ptr(page), 0);
+
 	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
 		clear_compound_head(p);
 		set_page_refcounted(p);
@@ -1287,6 +1290,9 @@ static void prep_compound_gigantic_page(struct page *page, unsigned int order)
 		set_compound_head(p, page);
 	}
 	atomic_set(compound_mapcount_ptr(page), -1);
+
+	if (hpage_pincount_available(page))
+		atomic_set(compound_pincount_ptr(page), 0);
 }
 
 /*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3c4eb750a199..b2fe61035b7a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -689,6 +689,8 @@ void prep_compound_page(struct page *page, unsigned int order)
 		set_compound_head(p, page);
 	}
 	atomic_set(compound_mapcount_ptr(page), -1);
+	if (hpage_pincount_available(page))
+		atomic_set(compound_pincount_ptr(page), 0);
 }
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
diff --git a/mm/rmap.c b/mm/rmap.c
index b3e381919835..e45b9b991e2f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1178,6 +1178,9 @@ void page_add_new_anon_rmap(struct page *page,
 		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 		/* increment count (starts at -1) */
 		atomic_set(compound_mapcount_ptr(page), 0);
+		if (hpage_pincount_available(page))
+			atomic_set(compound_pincount_ptr(page), 0);
+
 		__inc_node_page_state(page, NR_ANON_THPS);
 	} else {
 		/* Anon THP always mapped first with PMD */
@@ -1974,6 +1977,9 @@ void hugepage_add_new_anon_rmap(struct page *page,
 {
 	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
 	atomic_set(compound_mapcount_ptr(page), 0);
+	if (hpage_pincount_available(page))
+		atomic_set(compound_pincount_ptr(page), 0);
+
 	__page_set_anon_rmap(page, vma, address, 1);
 }
 #endif /* CONFIG_HUGETLB_PAGE */
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
                   ` (4 preceding siblings ...)
  2020-01-29  3:24 ` [PATCH v2 5/8] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29  3:24 ` [PATCH v2 7/8] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
  2020-01-29  3:24 ` [PATCH v2 8/8] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
  7 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
unpin_user_pages*(), we need some visibility into whether all of this is
working correctly.

Add two new fields to /proc/vmstat:

    nr_foll_pin_requested
    nr_foll_pin_returned

These are documented in Documentation/core-api/pin_user_pages.rst.
They represent the number of pages (since boot time) that have been
pinned ("nr_foll_pin_requested") and unpinned ("nr_foll_pin_returned"),
via pin_user_pages*() and unpin_user_pages*().

In the absence of long-running DMA or RDMA operations that hold pages
pinned, the above two fields will normally be equal to each other.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mmzone.h |  2 ++
 mm/gup.c               | 21 +++++++++++++++++++++
 mm/vmstat.c            |  2 ++
 3 files changed, 25 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..392868bc4763 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -243,6 +243,8 @@ enum node_stat_item {
 	NR_DIRTIED,		/* page dirtyings since bootup */
 	NR_WRITTEN,		/* page writings since bootup */
 	NR_KERNEL_MISC_RECLAIMABLE,	/* reclaimable non-slab kernel pages */
+	NR_FOLL_PIN_REQUESTED,	/* via: pin_user_page(), gup flag: FOLL_PIN */
+	NR_FOLL_PIN_RETURNED,	/* pages returned via unpin_user_page() */
 	NR_VM_NODE_STAT_ITEMS
 };
 
diff --git a/mm/gup.c b/mm/gup.c
index 03e7a5cfa6a9..d536bda383c4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -29,6 +29,19 @@ struct follow_page_context {
 	unsigned int page_mask;
 };
 
+#ifdef CONFIG_DEBUG_VM
+static inline void __update_proc_vmstat(struct page *page,
+					enum node_stat_item item, int count)
+{
+	mod_node_page_state(page_pgdat(page), item, count);
+}
+#else
+static inline void __update_proc_vmstat(struct page *page,
+					enum node_stat_item item, int count)
+{
+}
+#endif
+
 static void hpage_pincount_add(struct page *page, int refs)
 {
 	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
@@ -86,6 +99,8 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 	if (flags & FOLL_GET)
 		return try_get_compound_head(page, refs);
 	else if (flags & FOLL_PIN) {
+		int orig_refs = refs;
+
 		/*
 		 * When pinning a compound page of order > 1 (which is what
 		 * hpage_pincount_available() checks for), use an exact count to
@@ -104,6 +119,7 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 		if (hpage_pincount_available(page))
 			hpage_pincount_add(page, refs);
 
+		__update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, orig_refs);
 		return page;
 	}
 
@@ -159,6 +175,8 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 		 * once, so that the page really is pinned.
 		 */
 		page_ref_add(page, refs);
+
+		__update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1);
 	}
 
 	return true;
@@ -179,6 +197,7 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
 
 	count = page_ref_sub_return(page, refs);
 
+	__update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
 	/*
 	 * devmap page refcounts are 1-based, rather than 0-based: if
 	 * refcount is 1, then the page is free and the refcount is
@@ -229,6 +248,8 @@ void unpin_user_page(struct page *page)
 
 	if (page_ref_sub_and_test(page, refs))
 		__put_page(page);
+
+	__update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
 }
 EXPORT_SYMBOL(unpin_user_page);
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..b56808bae1b4 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1168,6 +1168,8 @@ const char * const vmstat_text[] = {
 	"nr_dirtied",
 	"nr_written",
 	"nr_kernel_misc_reclaimable",
+	"nr_foll_pin_requested",
+	"nr_foll_pin_returned",
 
 	/* enum writeback_stat_item counters */
 	"nr_dirty_threshold",
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 7/8] mm/gup_benchmark: support pin_user_pages() and related calls
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
                   ` (5 preceding siblings ...)
  2020-01-29  3:24 ` [PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  2020-01-29  3:24 ` [PATCH v2 8/8] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
  7 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

Up until now, gup_benchmark supported testing of the
following kernel functions:

* get_user_pages(): via the '-U' command line option
* get_user_pages_longterm(): via the '-L' command line option
* get_user_pages_fast(): as the default (no options required)

Add test coverage for the new corresponding pin_*() functions:

* pin_user_pages_fast(): via the '-a' command line option
* pin_user_pages():      via the '-b' command line option

Also, add an option for clarity: '-u' for what is now (still) the
default choice: get_user_pages_fast().

Also, for the commands that set FOLL_PIN, verify that the pages
really are dma-pinned, via the new is_dma_pinned() routine.
Those commands are:

    PIN_FAST_BENCHMARK     : calls pin_user_pages_fast()
    PIN_BENCHMARK          : calls pin_user_pages()

In between the calls to pin_*() and unpin_user_pages(),
check each page: if page_dma_pinned() returns false, then
WARN and return.

Do this outside of the benchmark timestamps, so that it doesn't
affect reported times.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup_benchmark.c                         | 71 ++++++++++++++++++++--
 tools/testing/selftests/vm/gup_benchmark.c | 15 ++++-
 2 files changed, 80 insertions(+), 6 deletions(-)

diff --git a/mm/gup_benchmark.c b/mm/gup_benchmark.c
index 8dba38e79a9f..7d5573083ab3 100644
--- a/mm/gup_benchmark.c
+++ b/mm/gup_benchmark.c
@@ -8,6 +8,8 @@
 #define GUP_FAST_BENCHMARK	_IOWR('g', 1, struct gup_benchmark)
 #define GUP_LONGTERM_BENCHMARK	_IOWR('g', 2, struct gup_benchmark)
 #define GUP_BENCHMARK		_IOWR('g', 3, struct gup_benchmark)
+#define PIN_FAST_BENCHMARK	_IOWR('g', 4, struct gup_benchmark)
+#define PIN_BENCHMARK		_IOWR('g', 5, struct gup_benchmark)
 
 struct gup_benchmark {
 	__u64 get_delta_usec;
@@ -19,6 +21,48 @@ struct gup_benchmark {
 	__u64 expansion[10];	/* For future use */
 };
 
+static void put_back_pages(unsigned int cmd, struct page **pages,
+			   unsigned long nr_pages)
+{
+	int i;
+
+	switch (cmd) {
+	case GUP_FAST_BENCHMARK:
+	case GUP_LONGTERM_BENCHMARK:
+	case GUP_BENCHMARK:
+		for (i = 0; i < nr_pages; i++)
+			put_page(pages[i]);
+		break;
+
+	case PIN_FAST_BENCHMARK:
+	case PIN_BENCHMARK:
+		unpin_user_pages(pages, nr_pages);
+		break;
+	}
+}
+
+static void verify_dma_pinned(unsigned int cmd, struct page **pages,
+			      unsigned long nr_pages)
+{
+	int i;
+	struct page *page;
+
+	switch (cmd) {
+	case PIN_FAST_BENCHMARK:
+	case PIN_BENCHMARK:
+		for (i = 0; i < nr_pages; i++) {
+			page = pages[i];
+			if (WARN(!page_dma_pinned(page),
+				 "pages[%d] is NOT dma-pinned\n", i)) {
+
+				dump_page(page, "gup_benchmark failure");
+				break;
+			}
+		}
+		break;
+	}
+}
+
 static int __gup_benchmark_ioctl(unsigned int cmd,
 		struct gup_benchmark *gup)
 {
@@ -66,6 +110,14 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 			nr = get_user_pages(addr, nr, gup->flags, pages + i,
 					    NULL);
 			break;
+		case PIN_FAST_BENCHMARK:
+			nr = pin_user_pages_fast(addr, nr, gup->flags,
+						 pages + i);
+			break;
+		case PIN_BENCHMARK:
+			nr = pin_user_pages(addr, nr, gup->flags, pages + i,
+					    NULL);
+			break;
 		default:
 			kvfree(pages);
 			ret = -EINVAL;
@@ -78,15 +130,22 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 	}
 	end_time = ktime_get();
 
+	/* Shifting the meaning of nr_pages: now it is actual number pinned: */
+	nr_pages = i;
+
 	gup->get_delta_usec = ktime_us_delta(end_time, start_time);
 	gup->size = addr - gup->addr;
 
+	/*
+	 * Take an un-benchmark-timed moment to verify DMA pinned
+	 * state: print a warning if any non-dma-pinned pages are found:
+	 */
+	verify_dma_pinned(cmd, pages, nr_pages);
+
 	start_time = ktime_get();
-	for (i = 0; i < nr_pages; i++) {
-		if (!pages[i])
-			break;
-		put_page(pages[i]);
-	}
+
+	put_back_pages(cmd, pages, nr_pages);
+
 	end_time = ktime_get();
 	gup->put_delta_usec = ktime_us_delta(end_time, start_time);
 
@@ -105,6 +164,8 @@ static long gup_benchmark_ioctl(struct file *filep, unsigned int cmd,
 	case GUP_FAST_BENCHMARK:
 	case GUP_LONGTERM_BENCHMARK:
 	case GUP_BENCHMARK:
+	case PIN_FAST_BENCHMARK:
+	case PIN_BENCHMARK:
 		break;
 	default:
 		return -EINVAL;
diff --git a/tools/testing/selftests/vm/gup_benchmark.c b/tools/testing/selftests/vm/gup_benchmark.c
index 389327e9b30a..43b4dfe161a2 100644
--- a/tools/testing/selftests/vm/gup_benchmark.c
+++ b/tools/testing/selftests/vm/gup_benchmark.c
@@ -18,6 +18,10 @@
 #define GUP_LONGTERM_BENCHMARK	_IOWR('g', 2, struct gup_benchmark)
 #define GUP_BENCHMARK		_IOWR('g', 3, struct gup_benchmark)
 
+/* Similar to above, but use FOLL_PIN instead of FOLL_GET. */
+#define PIN_FAST_BENCHMARK	_IOWR('g', 4, struct gup_benchmark)
+#define PIN_BENCHMARK		_IOWR('g', 5, struct gup_benchmark)
+
 /* Just the flags we need, copied from mm.h: */
 #define FOLL_WRITE	0x01	/* check pte is writable */
 
@@ -40,8 +44,14 @@ int main(int argc, char **argv)
 	char *file = "/dev/zero";
 	char *p;
 
-	while ((opt = getopt(argc, argv, "m:r:n:f:tTLUwSH")) != -1) {
+	while ((opt = getopt(argc, argv, "m:r:n:f:abtTLUuwSH")) != -1) {
 		switch (opt) {
+		case 'a':
+			cmd = PIN_FAST_BENCHMARK;
+			break;
+		case 'b':
+			cmd = PIN_BENCHMARK;
+			break;
 		case 'm':
 			size = atoi(optarg) * MB;
 			break;
@@ -63,6 +73,9 @@ int main(int argc, char **argv)
 		case 'U':
 			cmd = GUP_BENCHMARK;
 			break;
+		case 'u':
+			cmd = GUP_FAST_BENCHMARK;
+			break;
 		case 'w':
 			write = 1;
 			break;
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* [PATCH v2 8/8] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage
  2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
                   ` (6 preceding siblings ...)
  2020-01-29  3:24 ` [PATCH v2 7/8] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
@ 2020-01-29  3:24 ` John Hubbard
  7 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-29  3:24 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Al Viro, Christoph Hellwig, Dan Williams, Dave Chinner,
	Ira Weiny, Jan Kara, Jason Gunthorpe, Jonathan Corbet,
	Jérôme Glisse, Kirill A . Shutemov, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	John Hubbard

It's good to have basic unit test coverage of the new FOLL_PIN
behavior. Fortunately, the gup_benchmark unit test is extremely
fast (a few milliseconds), so adding it the the run_vmtests suite
is going to cause no noticeable change in running time.

So, add two new invocations to run_vmtests:

1) Run gup_benchmark with normal get_user_pages().

2) Run gup_benchmark with pin_user_pages(). This is much like
the first call, except that it sets FOLL_PIN.

Running these two in quick succession also provide a visual
comparison of the running times, which is convenient.

The new invocations are fairly early in the run_vmtests script,
because with test suites, it's usually preferable to put the
shorter, faster tests first, all other things being equal.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 tools/testing/selftests/vm/run_vmtests | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/testing/selftests/vm/run_vmtests b/tools/testing/selftests/vm/run_vmtests
index a692ea828317..df6a6bf3f238 100755
--- a/tools/testing/selftests/vm/run_vmtests
+++ b/tools/testing/selftests/vm/run_vmtests
@@ -112,6 +112,28 @@ echo "NOTE: The above hugetlb tests provide minimal coverage.  Use"
 echo "      https://github.com/libhugetlbfs/libhugetlbfs.git for"
 echo "      hugetlb regression testing."
 
+echo "--------------------------------------------"
+echo "running 'gup_benchmark -U' (normal/slow gup)"
+echo "--------------------------------------------"
+./gup_benchmark -U
+if [ $? -ne 0 ]; then
+	echo "[FAIL]"
+	exitcode=1
+else
+	echo "[PASS]"
+fi
+
+echo "------------------------------------------"
+echo "running gup_benchmark -b (pin_user_pages)"
+echo "------------------------------------------"
+./gup_benchmark -b
+if [ $? -ne 0 ]; then
+	echo "[FAIL]"
+	exitcode=1
+else
+	echo "[PASS]"
+fi
+
 echo "-------------------"
 echo "running userfaultfd"
 echo "-------------------"
-- 
2.25.0


^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
  2020-01-29  3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
@ 2020-01-29 11:25   ` Kirill A. Shutemov
  2020-01-29 22:26     ` John Hubbard
  0 siblings, 1 reply; 18+ messages in thread
From: Kirill A. Shutemov @ 2020-01-29 11:25 UTC (permalink / raw)
  To: John Hubbard
  Cc: Andrew Morton, Al Viro, Christoph Hellwig, Dan Williams,
	Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML

On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
> When debugging a problem that involves compound pages, it is extremely
> helpful if dump_page() reports not only the page->_refcount, but also
> the refcount of the head page of the compound page. That's because the
> head page collects refcounts for the entire compound page.
> 
> Therefore, enhance dump_page() so as to print out the refcount of the
> head page of a compound page.
> 
> This approach (printing information about a struct page that is not the
> struct page that was passed into dump_page()) has a precedent:
> compound_mapcount is already being printed.

refcount on a tail must always be 0. I think we should only print it when
it is non-zero, emphasizing this fact with a standalone message.

-- 
 Kirill A. Shutemov

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages
  2020-01-29  3:24 ` [PATCH v2 4/8] mm/gup: track FOLL_PIN pages John Hubbard
@ 2020-01-29 13:51   ` Kirill A. Shutemov
  2020-01-30  6:44     ` John Hubbard
  0 siblings, 1 reply; 18+ messages in thread
From: Kirill A. Shutemov @ 2020-01-29 13:51 UTC (permalink / raw)
  To: John Hubbard
  Cc: Andrew Morton, Al Viro, Christoph Hellwig, Dan Williams,
	Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	Kirill A . Shutemov

On Tue, Jan 28, 2020 at 07:24:13PM -0800, John Hubbard wrote:
> Add tracking of pages that were pinned via FOLL_PIN. This tracking is
> implemented via overloading of page->_refcount: pins are added by
> adding GUP_PIN_COUNTING_BIAS (1024) to the refcount. This provides a
> fuzzy indication of pinning, and it can have false positives (and that's
> OK). Please see the pre-existing
> Documentation/core-api/pin_user_pages.rst for details.
> 
> As mentioned in pin_user_pages.rst, callers who effectively set FOLL_PIN
> (typically via pin_user_pages*()) are required to ultimately free such
> pages via unpin_user_page().
> 
> Please also not the limitation, discussed in pin_user_pages.rst under

s/not/note/

> the "TODO: for 1GB and larger huge pages" section. (That limitation will
> be removed in a following patch.)
> 
> The effect of a FOLL_PIN flag is similar to that of FOLL_GET, and may be
> thought of as "FOLL_GET for DIO and/or RDMA use".
> 
> Pages that have been pinned via FOLL_PIN are identifiable via a
> new function call:
> 
>    bool page_dma_pinned(struct page *page);
> 
> What to do in response to encountering such a page, is left to later
> patchsets. There is discussion about this in [1], [2], [3], and [4].
> 
> This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().
> 
> [1] Some slow progress on get_user_pages() (Apr 2, 2019):
>     https://lwn.net/Articles/784574/
> [2] DMA and get_user_pages() (LPC: Dec 12, 2018):
>     https://lwn.net/Articles/774411/
> [3] The trouble with get_user_pages() (Apr 30, 2018):
>     https://lwn.net/Articles/753027/
> [4] LWN kernel index: get_user_pages():
>     https://lwn.net/Kernel/Index/#Memory_management-get_user_pages
> 
> Suggested-by: Jan Kara <jack@suse.cz>
> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> 
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  include/linux/mm.h       |  83 +++++++++--
>  include/linux/page_ref.h |  10 ++
>  mm/gup.c                 | 293 +++++++++++++++++++++++++++++++--------
>  mm/huge_memory.c         |  29 +++-
>  mm/hugetlb.c             |  38 ++---
>  5 files changed, 354 insertions(+), 99 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 080f8ac8bfb7..c5d0f4a66788 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1001,6 +1001,8 @@ static inline void get_page(struct page *page)
>  	page_ref_inc(page);
>  }
>  
> +bool __must_check try_grab_page(struct page *page, unsigned int flags);
> +
>  static inline __must_check bool try_get_page(struct page *page)
>  {
>  	page = compound_head(page);
> @@ -1029,29 +1031,80 @@ static inline void put_page(struct page *page)
>  		__put_page(page);
>  }
>  
> -/**
> - * unpin_user_page() - release a gup-pinned page
> - * @page:            pointer to page to be released
> +/*
> + * GUP_PIN_COUNTING_BIAS, and the associated functions that use it, overload
> + * the page's refcount so that two separate items are tracked: the original page
> + * reference count, and also a new count of how many pin_user_pages() calls were
> + * made against the page. ("gup-pinned" is another term for the latter).
> + *
> + * With this scheme, pin_user_pages() becomes special: such pages are marked as
> + * distinct from normal pages. As such, the unpin_user_page() call (and its
> + * variants) must be used in order to release gup-pinned pages.
> + *
> + * Choice of value:
> + *
> + * By making GUP_PIN_COUNTING_BIAS a power of two, debugging of page reference
> + * counts with respect to pin_user_pages() and unpin_user_page() becomes
> + * simpler, due to the fact that adding an even power of two to the page
> + * refcount has the effect of using only the upper N bits, for the code that
> + * counts up using the bias value. This means that the lower bits are left for
> + * the exclusive use of the original code that increments and decrements by one
> + * (or at least, by much smaller values than the bias value).
>   *
> - * Pages that were pinned via pin_user_pages*() must be released via either
> - * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
> - * that eventually such pages can be separately tracked and uniquely handled. In
> - * particular, interactions with RDMA and filesystems need special handling.
> + * Of course, once the lower bits overflow into the upper bits (and this is
> + * OK, because subtraction recovers the original values), then visual inspection
> + * no longer suffices to directly view the separate counts. However, for normal
> + * applications that don't have huge page reference counts, this won't be an
> + * issue.
>   *
> - * unpin_user_page() and put_page() are not interchangeable, despite this early
> - * implementation that makes them look the same. unpin_user_page() calls must
> - * be perfectly matched up with pin*() calls.
> + * Locking: the lockless algorithm described in page_cache_get_speculative()
> + * and page_cache_gup_pin_speculative() provides safe operation for
> + * get_user_pages and page_mkclean and other calls that race to set up page
> + * table entries.
>   */
> -static inline void unpin_user_page(struct page *page)
> -{
> -	put_page(page);
> -}
> +#define GUP_PIN_COUNTING_BIAS (1U << 10)
>  
> +void unpin_user_page(struct page *page);
>  void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>  				 bool make_dirty);
> -
>  void unpin_user_pages(struct page **pages, unsigned long npages);
>  
> +/**
> + * page_dma_pinned() - report if a page is pinned for DMA.
> + *
> + * This function checks if a page has been pinned via a call to
> + * pin_user_pages*().
> + *
> + * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
> + * because it means "definitely not pinned for DMA", but true means "probably
> + * pinned for DMA, but possibly a false positive due to having at least
> + * GUP_PIN_COUNTING_BIAS worth of normal page references".
> + *
> + * False positives are OK, because: a) it's unlikely for a page to get that many
> + * refcounts, and b) all the callers of this routine are expected to be able to
> + * deal gracefully with a false positive.

I wounder if we should reverse the logic and name -- page_not_dma_pinned()
or something -- too emphasise that we can only know for sure when the page
is not pinned, but not necessary when it is.

> + *
> + * For more information, please see Documentation/vm/pin_user_pages.rst.
> + *
> + * @page:	pointer to page to be queried.
> + * @Return:	True, if it is likely that the page has been "dma-pinned".
> + *		False, if the page is definitely not dma-pinned.
> + */
> +static inline bool page_dma_pinned(struct page *page)
> +{
> +	/*
> +	 * page_ref_count() is signed. If that refcount overflows, then
> +	 * page_ref_count() returns a negative value, and callers will avoid
> +	 * further incrementing the refcount.
> +	 *
> +	 * Here, for that overflow case, use the signed bit to count a little
> +	 * bit higher via unsigned math, and thus still get an accurate result
> +	 * from page_dma_pinned().
> +	 */
> +	return ((unsigned int)page_ref_count(compound_head(page))) >=
> +		GUP_PIN_COUNTING_BIAS;

Do you expect it too be called on tail pages?

> +}
> +
>  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>  #define SECTION_IN_PAGE_FLAGS
>  #endif
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 14d14beb1f7f..b9cbe553d1e7 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -102,6 +102,16 @@ static inline void page_ref_sub(struct page *page, int nr)
>  		__page_ref_mod(page, -nr);
>  }
>  
> +static inline int page_ref_sub_return(struct page *page, int nr)
> +{
> +	int ret = atomic_sub_return(nr, &page->_refcount);
> +
> +	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> +		__page_ref_mod(page, -nr);
> +
> +	return ret;
> +}
> +

I see opportunity to split the patch further.

>  static inline void page_ref_inc(struct page *page)
>  {
>  	atomic_inc(&page->_refcount);
> diff --git a/mm/gup.c b/mm/gup.c
> index 9e117998274c..7a96490dcc54 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -44,6 +44,136 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
>  	return head;
>  }
>  
> +/*
> + * try_grab_compound_head() - attempt to elevate a page's refcount, by a
> + * flags-dependent amount.
> + *
> + * "grab" names in this file mean, "look at flags to decide whether to use
> + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
> + *
> + * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
> + * same time. (That's true throughout the get_user_pages*() and
> + * pin_user_pages*() APIs.) Cases:
> + *
> + *    FOLL_GET: page's refcount will be incremented by 1.
> + *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
> + *
> + * Return: head page (with refcount appropriately incremented) for success, or
> + * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
> + * considered failure, and furthermore, a likely bug in the caller, so a warning
> + * is also emitted.
> + */
> +static __maybe_unused struct page *try_grab_compound_head(struct page *page,
> +							  int refs,
> +							  unsigned int flags)
> +{
> +	if (flags & FOLL_GET)
> +		return try_get_compound_head(page, refs);
> +	else if (flags & FOLL_PIN) {
> +		refs *= GUP_PIN_COUNTING_BIAS;
> +		return try_get_compound_head(page, refs);

Maybe overflow detection? At least under VM_BUG_ON()?

> +	}
> +
> +	WARN_ON_ONCE(1);
> +	return NULL;
> +}
> +
> +/**
> + * try_grab_page() - elevate a page's refcount by a flag-dependent amount
> + *
> + * This might not do anything at all, depending on the flags argument.
> + *
> + * "grab" names in this file mean, "look at flags to decide whether to use
> + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
> + *
> + * @page:    pointer to page to be grabbed
> + * @flags:   gup flags: these are the FOLL_* flag values.
> + *
> + * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
> + * time. Cases:
> + *
> + *    FOLL_GET: page's refcount will be incremented by 1.
> + *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
> + *
> + * Return: true for success, or if no action was required (if neither FOLL_PIN
> + * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
> + * FOLL_PIN was set, but the page could not be grabbed.
> + */
> +bool __must_check try_grab_page(struct page *page, unsigned int flags)
> +{
> +	if (flags & FOLL_GET)
> +		return try_get_page(page);
> +	else if (flags & FOLL_PIN) {
> +		page = compound_head(page);
> +
> +		if (WARN_ON_ONCE(flags & FOLL_GET))

This is always false -- you won't get to the branch if FOLL_GET is set.

Maybe

	WARN_ON_ONCE(flags & (FOLL_GET|FOLL_PIN) == FOLL_GET|FOLL_PIN);

at the start of the function?

> +			return false;
> +
> +		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
> +			return false;
> +
> +		page_ref_add(page, GUP_PIN_COUNTING_BIAS);
> +	}
> +
> +	return true;
> +}
> +
> +#ifdef CONFIG_DEV_PAGEMAP_OPS
> +static bool __unpin_devmap_managed_user_page(struct page *page)
> +{
> +	int count;
> +
> +	if (!page_is_devmap_managed(page))
> +		return false;
> +
> +	count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
> +
> +	/*
> +	 * devmap page refcounts are 1-based, rather than 0-based: if
> +	 * refcount is 1, then the page is free and the refcount is
> +	 * stable because nobody holds a reference on the page.
> +	 */
> +	if (count == 1)
> +		free_devmap_managed_page(page);
> +	else if (!count)
> +		__put_page(page);
> +
> +	return true;
> +}
> +#else
> +static bool __unpin_devmap_managed_user_page(struct page *page)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_DEV_PAGEMAP_OPS */
> +
> +/**
> + * unpin_user_page() - release a dma-pinned page
> + * @page:            pointer to page to be released
> + *
> + * Pages that were pinned via pin_user_pages*() must be released via either
> + * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
> + * that such pages can be separately tracked and uniquely handled. In
> + * particular, interactions with RDMA and filesystems need special handling.
> + */
> +void unpin_user_page(struct page *page)
> +{
> +	page = compound_head(page);
> +
> +	/*
> +	 * For devmap managed pages we need to catch refcount transition from
> +	 * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
> +	 * page is free and we need to inform the device driver through
> +	 * callback. See include/linux/memremap.h and HMM for details.
> +	 */
> +	if (__unpin_devmap_managed_user_page(page))
> +		return;
> +
> +	if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
> +		__put_page(page);
> +}
> +EXPORT_SYMBOL(unpin_user_page);
> +
>  /**
>   * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
>   * @pages:  array of pages to be maybe marked dirty, and definitely released.
> @@ -230,10 +360,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  	}
>  
>  	page = vm_normal_page(vma, address, pte);
> -	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
> +	if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
>  		/*
> -		 * Only return device mapping pages in the FOLL_GET case since
> -		 * they are only valid while holding the pgmap reference.
> +		 * Only return device mapping pages in the FOLL_GET or FOLL_PIN
> +		 * case since they are only valid while holding the pgmap
> +		 * reference.
>  		 */
>  		*pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);
>  		if (*pgmap)
> @@ -271,11 +402,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  		goto retry;
>  	}
>  
> -	if (flags & FOLL_GET) {
> -		if (unlikely(!try_get_page(page))) {
> -			page = ERR_PTR(-ENOMEM);
> -			goto out;
> -		}
> +	/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
> +	if (unlikely(!try_grab_page(page, flags))) {
> +		page = ERR_PTR(-ENOMEM);
> +		goto out;
>  	}
>  	if (flags & FOLL_TOUCH) {
>  		if ((flags & FOLL_WRITE) &&
> @@ -537,7 +667,7 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
>  	/* make this handle hugepd */
>  	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
>  	if (!IS_ERR(page)) {
> -		BUG_ON(flags & FOLL_GET);
> +		WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
>  		return page;
>  	}
>  
> @@ -1675,6 +1805,15 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
>  {
>  	return 0;
>  }
> +
> +static long __get_user_pages_remote(struct task_struct *tsk,
> +				    struct mm_struct *mm,
> +				    unsigned long start, unsigned long nr_pages,
> +				    unsigned int gup_flags, struct page **pages,
> +				    struct vm_area_struct **vmas, int *locked)
> +{
> +	return 0;
> +}
>  #endif /* !CONFIG_MMU */
>  
>  /*
> @@ -1870,13 +2009,17 @@ static inline pte_t gup_get_pte(pte_t *ptep)
>  #endif /* CONFIG_GUP_GET_PTE_LOW_HIGH */
>  
>  static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
> +					    unsigned int flags,
>  					    struct page **pages)
>  {
>  	while ((*nr) - nr_start) {
>  		struct page *page = pages[--(*nr)];
>  
>  		ClearPageReferenced(page);
> -		put_page(page);
> +		if (flags & FOLL_PIN)
> +			unpin_user_page(page);
> +		else
> +			put_page(page);
>  	}
>  }
>  
> @@ -1909,7 +2052,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  
>  			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
>  			if (unlikely(!pgmap)) {
> -				undo_dev_pagemap(nr, nr_start, pages);
> +				undo_dev_pagemap(nr, nr_start, flags, pages);
>  				goto pte_unmap;
>  			}
>  		} else if (pte_special(pte))
> @@ -1918,7 +2061,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  		page = pte_page(pte);
>  
> -		head = try_get_compound_head(page, 1);
> +		head = try_grab_compound_head(page, 1, flags);
>  		if (!head)
>  			goto pte_unmap;
>  
> @@ -1974,12 +2117,15 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
>  
>  		pgmap = get_dev_pagemap(pfn, pgmap);
>  		if (unlikely(!pgmap)) {
> -			undo_dev_pagemap(nr, nr_start, pages);
> +			undo_dev_pagemap(nr, nr_start, flags, pages);
>  			return 0;
>  		}
>  		SetPageReferenced(page);
>  		pages[*nr] = page;
> -		get_page(page);
> +		if (unlikely(!try_grab_page(page, flags))) {
> +			undo_dev_pagemap(nr, nr_start, flags, pages);
> +			return 0;
> +		}
>  		(*nr)++;
>  		pfn++;
>  	} while (addr += PAGE_SIZE, addr != end);
> @@ -2001,7 +2147,7 @@ static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>  		return 0;
>  
>  	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> -		undo_dev_pagemap(nr, nr_start, pages);
> +		undo_dev_pagemap(nr, nr_start, flags, pages);
>  		return 0;
>  	}
>  	return 1;
> @@ -2019,7 +2165,7 @@ static int __gup_device_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>  		return 0;
>  
>  	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
> -		undo_dev_pagemap(nr, nr_start, pages);
> +		undo_dev_pagemap(nr, nr_start, flags, pages);
>  		return 0;
>  	}
>  	return 1;
> @@ -2053,8 +2199,11 @@ static int record_subpages(struct page *page, unsigned long addr,
>  	return nr;
>  }
>  
> -static void put_compound_head(struct page *page, int refs)
> +static void put_compound_head(struct page *page, int refs, unsigned int flags)
>  {
> +	if (flags & FOLL_PIN)
> +		refs *= GUP_PIN_COUNTING_BIAS;
> +
>  	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
>  	/*
>  	 * Calling put_page() for each ref is unnecessarily slow. Only the last
> @@ -2098,12 +2247,12 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
>  	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
>  	refs = record_subpages(page, addr, end, pages + *nr);
>  
> -	head = try_get_compound_head(head, refs);
> +	head = try_grab_compound_head(head, refs, flags);
>  	if (!head)
>  		return 0;
>  
>  	if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> -		put_compound_head(head, refs);
> +		put_compound_head(head, refs, flags);
>  		return 0;
>  	}
>  
> @@ -2158,12 +2307,12 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>  	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>  	refs = record_subpages(page, addr, end, pages + *nr);
>  
> -	head = try_get_compound_head(pmd_page(orig), refs);
> +	head = try_grab_compound_head(pmd_page(orig), refs, flags);
>  	if (!head)
>  		return 0;
>  
>  	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> -		put_compound_head(head, refs);
> +		put_compound_head(head, refs, flags);
>  		return 0;
>  	}
>  
> @@ -2192,12 +2341,12 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>  	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
>  	refs = record_subpages(page, addr, end, pages + *nr);
>  
> -	head = try_get_compound_head(pud_page(orig), refs);
> +	head = try_grab_compound_head(pud_page(orig), refs, flags);
>  	if (!head)
>  		return 0;
>  
>  	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
> -		put_compound_head(head, refs);
> +		put_compound_head(head, refs, flags);
>  		return 0;
>  	}
>  
> @@ -2221,12 +2370,12 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
>  	page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
>  	refs = record_subpages(page, addr, end, pages + *nr);
>  
> -	head = try_get_compound_head(pgd_page(orig), refs);
> +	head = try_grab_compound_head(pgd_page(orig), refs, flags);
>  	if (!head)
>  		return 0;
>  
>  	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
> -		put_compound_head(head, refs);
> +		put_compound_head(head, refs, flags);
>  		return 0;
>  	}
>  
> @@ -2389,6 +2538,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	unsigned long len, end;
>  	unsigned long flags;
>  	int nr = 0;
> +	/*
> +	 * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
> +	 * because gup fast is always a "pin with a +1 page refcount" request.
> +	 */
> +	unsigned int gup_flags = FOLL_GET;
> +
> +	if (write)
> +		gup_flags |= FOLL_WRITE;
>  
>  	start = untagged_addr(start) & PAGE_MASK;
>  	len = (unsigned long) nr_pages << PAGE_SHIFT;
> @@ -2414,7 +2571,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
>  	    gup_fast_permitted(start, end)) {
>  		local_irq_save(flags);
> -		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
> +		gup_pgd_range(start, end, gup_flags, pages, &nr);
>  		local_irq_restore(flags);
>  	}
>  
> @@ -2453,7 +2610,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
>  	int nr = 0, ret = 0;
>  
>  	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
> -				       FOLL_FORCE | FOLL_PIN)))
> +				       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
>  		return -EINVAL;
>  
>  	start = untagged_addr(start) & PAGE_MASK;
> @@ -2496,11 +2653,11 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
>  
>  /**
>   * get_user_pages_fast() - pin user pages in memory
> - * @start:	starting user address
> - * @nr_pages:	number of pages from start to pin
> - * @gup_flags:	flags modifying pin behaviour
> - * @pages:	array that receives pointers to the pages pinned.
> - *		Should be at least nr_pages long.
> + * @start:      starting user address
> + * @nr_pages:   number of pages from start to pin
> + * @gup_flags:  flags modifying pin behaviour
> + * @pages:      array that receives pointers to the pages pinned.
> + *              Should be at least nr_pages long.
>   *
>   * Attempt to pin user pages in memory without taking mm->mmap_sem.
>   * If not successful, it will fall back to taking the lock and
> @@ -2520,6 +2677,13 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>  	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
>  		return -EINVAL;
>  
> +	/*
> +	 * The caller may or may not have explicitly set FOLL_GET; either way is
> +	 * OK. However, internally (within mm/gup.c), gup fast variants must set
> +	 * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
> +	 * request.
> +	 */
> +	gup_flags |= FOLL_GET;
>  	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
>  }
>  EXPORT_SYMBOL_GPL(get_user_pages_fast);
> @@ -2527,9 +2691,12 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
>  /**
>   * pin_user_pages_fast() - pin user pages in memory without taking locks
>   *
> - * For now, this is a placeholder function, until various call sites are
> - * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
> - * this is identical to get_user_pages_fast().
> + * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
> + * get_user_pages_fast() for documentation on the function arguments, because
> + * the arguments here are identical.
> + *
> + * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
> + * see Documentation/vm/pin_user_pages.rst for further details.
>   *
>   * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
>   * is NOT intended for Case 2 (RDMA: long-term pins).
> @@ -2537,21 +2704,24 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
>  int pin_user_pages_fast(unsigned long start, int nr_pages,
>  			unsigned int gup_flags, struct page **pages)
>  {
> -	/*
> -	 * This is a placeholder, until the pin functionality is activated.
> -	 * Until then, just behave like the corresponding get_user_pages*()
> -	 * routine.
> -	 */
> -	return get_user_pages_fast(start, nr_pages, gup_flags, pages);
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> +		return -EINVAL;
> +
> +	gup_flags |= FOLL_PIN;
> +	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
>  }
>  EXPORT_SYMBOL_GPL(pin_user_pages_fast);
>  
>  /**
>   * pin_user_pages_remote() - pin pages of a remote process (task != current)
>   *
> - * For now, this is a placeholder function, until various call sites are
> - * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
> - * this is identical to get_user_pages_remote().
> + * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
> + * get_user_pages_remote() for documentation on the function arguments, because
> + * the arguments here are identical.
> + *
> + * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
> + * see Documentation/vm/pin_user_pages.rst for details.
>   *
>   * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
>   * is NOT intended for Case 2 (RDMA: long-term pins).
> @@ -2561,22 +2731,24 @@ long pin_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
>  			   unsigned int gup_flags, struct page **pages,
>  			   struct vm_area_struct **vmas, int *locked)
>  {
> -	/*
> -	 * This is a placeholder, until the pin functionality is activated.
> -	 * Until then, just behave like the corresponding get_user_pages*()
> -	 * routine.
> -	 */
> -	return get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags, pages,
> -				     vmas, locked);
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> +		return -EINVAL;
> +
> +	gup_flags |= FOLL_PIN;
> +	return __get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags,
> +				       pages, vmas, locked);
>  }
>  EXPORT_SYMBOL(pin_user_pages_remote);
>  
>  /**
>   * pin_user_pages() - pin user pages in memory for use by other devices
>   *
> - * For now, this is a placeholder function, until various call sites are
> - * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
> - * this is identical to get_user_pages().
> + * Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
> + * FOLL_PIN is set.
> + *
> + * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
> + * see Documentation/vm/pin_user_pages.rst for details.
>   *
>   * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
>   * is NOT intended for Case 2 (RDMA: long-term pins).
> @@ -2585,11 +2757,12 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
>  		    unsigned int gup_flags, struct page **pages,
>  		    struct vm_area_struct **vmas)
>  {
> -	/*
> -	 * This is a placeholder, until the pin functionality is activated.
> -	 * Until then, just behave like the corresponding get_user_pages*()
> -	 * routine.
> -	 */
> -	return get_user_pages(start, nr_pages, gup_flags, pages, vmas);
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> +		return -EINVAL;
> +
> +	gup_flags |= FOLL_PIN;
> +	return __gup_longterm_locked(current, current->mm, start, nr_pages,
> +				     pages, vmas, gup_flags);
>  }
>  EXPORT_SYMBOL(pin_user_pages);
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0a55dec68925..b1079aaa6f24 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>  	 */
>  	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
>  
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> +			 (FOLL_PIN | FOLL_GET)))

Too many parentheses.

> +		return NULL;
> +
>  	if (flags & FOLL_WRITE && !pmd_write(*pmd))
>  		return NULL;
>  
> @@ -973,7 +978,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>  	 * device mapped pages can only be returned if the
>  	 * caller will manage the page reference count.
>  	 */
> -	if (!(flags & FOLL_GET))
> +	if (!(flags & (FOLL_GET | FOLL_PIN)))
>  		return ERR_PTR(-EEXIST);
>  
>  	pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
> @@ -981,7 +986,8 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>  	if (!*pgmap)
>  		return ERR_PTR(-EFAULT);
>  	page = pfn_to_page(pfn);
> -	get_page(page);
> +	if (!try_grab_page(page, flags))
> +		page = ERR_PTR(-ENOMEM);
>  
>  	return page;
>  }
> @@ -1101,6 +1107,11 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
>  	if (flags & FOLL_WRITE && !pud_write(*pud))
>  		return NULL;
>  
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> +			 (FOLL_PIN | FOLL_GET)))
> +		return NULL;
> +

Ditto.

>  	if (pud_present(*pud) && pud_devmap(*pud))
>  		/* pass */;
>  	else
> @@ -1112,8 +1123,10 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
>  	/*
>  	 * device mapped pages can only be returned if the
>  	 * caller will manage the page reference count.
> +	 *
> +	 * At least one of FOLL_GET | FOLL_PIN must be set, so assert that here:
>  	 */
> -	if (!(flags & FOLL_GET))
> +	if (!(flags & (FOLL_GET | FOLL_PIN)))
>  		return ERR_PTR(-EEXIST);
>  
>  	pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
> @@ -1121,7 +1134,8 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
>  	if (!*pgmap)
>  		return ERR_PTR(-EFAULT);
>  	page = pfn_to_page(pfn);
> -	get_page(page);
> +	if (!try_grab_page(page, flags))
> +		page = ERR_PTR(-ENOMEM);
>  
>  	return page;
>  }
> @@ -1497,8 +1511,13 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>  
>  	page = pmd_page(*pmd);
>  	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
> +
> +	if (!try_grab_page(page, flags))
> +		return ERR_PTR(-ENOMEM);
> +
>  	if (flags & FOLL_TOUCH)
>  		touch_pmd(vma, addr, pmd, flags);
> +
>  	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
>  		/*
>  		 * We don't mlock() pte-mapped THPs. This way we can avoid
> @@ -1535,8 +1554,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>  skip_mlock:
>  	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
>  	VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
> -	if (flags & FOLL_GET)
> -		get_page(page);
>  
>  out:
>  	return page;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index dd8737a94bec..487e998fd38e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4375,19 +4375,6 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
>  		page = pte_page(huge_ptep_get(pte));
>  
> -		/*
> -		 * Instead of doing 'try_get_page()' below in the same_page
> -		 * loop, just check the count once here.
> -		 */
> -		if (unlikely(page_count(page) <= 0)) {
> -			if (pages) {
> -				spin_unlock(ptl);
> -				remainder = 0;
> -				err = -ENOMEM;
> -				break;
> -			}
> -		}
> -
>  		/*
>  		 * If subpage information not requested, update counters
>  		 * and skip the same_page loop below.
> @@ -4405,7 +4392,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  same_page:
>  		if (pages) {
>  			pages[i] = mem_map_offset(page, pfn_offset);
> -			get_page(pages[i]);
> +			if (!try_grab_page(pages[i], flags)) {
> +				spin_unlock(ptl);
> +				remainder = 0;
> +				err = -ENOMEM;
> +				WARN_ON_ONCE(1);
> +				break;
> +			}
>  		}
>  
>  		if (vmas)
> @@ -4965,6 +4958,12 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  	struct page *page = NULL;
>  	spinlock_t *ptl;
>  	pte_t pte;
> +
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> +			 (FOLL_PIN | FOLL_GET)))
> +		return NULL;
> +

Ditto.

>  retry:
>  	ptl = pmd_lockptr(mm, pmd);
>  	spin_lock(ptl);
> @@ -4977,8 +4976,11 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  	pte = huge_ptep_get((pte_t *)pmd);
>  	if (pte_present(pte)) {
>  		page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
> -		if (flags & FOLL_GET)
> -			get_page(page);
> +		if (unlikely(!try_grab_page(page, flags))) {
> +			WARN_ON_ONCE(1);
> +			page = NULL;
> +			goto out;
> +		}
>  	} else {
>  		if (is_hugetlb_entry_migration(pte)) {
>  			spin_unlock(ptl);
> @@ -4999,7 +5001,7 @@ struct page * __weak
>  follow_huge_pud(struct mm_struct *mm, unsigned long address,
>  		pud_t *pud, int flags)
>  {
> -	if (flags & FOLL_GET)
> +	if (flags & (FOLL_GET | FOLL_PIN))
>  		return NULL;
>  
>  	return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> @@ -5008,7 +5010,7 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address,
>  struct page * __weak
>  follow_huge_pgd(struct mm_struct *mm, unsigned long address, pgd_t *pgd, int flags)
>  {
> -	if (flags & FOLL_GET)
> +	if (flags & (FOLL_GET | FOLL_PIN))
>  		return NULL;
>  
>  	return pte_page(*(pte_t *)pgd) + ((address & ~PGDIR_MASK) >> PAGE_SHIFT);
> -- 
> 2.25.0
> 

-- 
 Kirill A. Shutemov

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
  2020-01-29 11:25   ` Kirill A. Shutemov
@ 2020-01-29 22:26     ` John Hubbard
  2020-01-29 22:59       ` Matthew Wilcox
  0 siblings, 1 reply; 18+ messages in thread
From: John Hubbard @ 2020-01-29 22:26 UTC (permalink / raw)
  To: Kirill A. Shutemov
  Cc: Andrew Morton, Al Viro, Christoph Hellwig, Dan Williams,
	Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML

On 1/29/20 3:25 AM, Kirill A. Shutemov wrote:
> On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
>> When debugging a problem that involves compound pages, it is extremely
>> helpful if dump_page() reports not only the page->_refcount, but also
>> the refcount of the head page of the compound page. That's because the
>> head page collects refcounts for the entire compound page.
>>
>> Therefore, enhance dump_page() so as to print out the refcount of the
>> head page of a compound page.
>>
>> This approach (printing information about a struct page that is not the
>> struct page that was passed into dump_page()) has a precedent:
>> compound_mapcount is already being printed.
> 
> refcount on a tail must always be 0. I think we should only print it when
> it is non-zero, emphasizing this fact with a standalone message.
> 

Hi Kirill,

Yes, good point, that sounds like just the right balance. And it avoids adding 
a new item to print in the common case, which is very nice. Here's what I've
changed it to for the next version (I'll also rewrite the commit description 
accordingly):


diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..3a45e2b77de0 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -75,12 +75,17 @@ void __dump_page(struct page *page, const char *reason)
 	 */
 	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
 
-	if (PageCompound(page))
-		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
-			"index:%#lx compound_mapcount: %d\n",
-			page, page_ref_count(page), mapcount,
+	if (PageCompound(page)) {
+		pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
+			"index:%#lx compound_mapcount:%d\n",
+			page, page_ref_count(compound_head(page)), mapcount,
 			page->mapping, page_to_pgoff(page),
 			compound_mapcount(page));
+
+		if (page != compound_head(page) && page_ref_count(page) != 0)
+			pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
+				"this tail page\n", page, page_ref_count(page));
+	}
 	else
 		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
 			page, page_ref_count(page), mapcount,


thanks,
-- 
John Hubbard
NVIDIA

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
  2020-01-29 22:26     ` John Hubbard
@ 2020-01-29 22:59       ` Matthew Wilcox
  2020-01-30  6:23         ` John Hubbard
  0 siblings, 1 reply; 18+ messages in thread
From: Matthew Wilcox @ 2020-01-29 22:59 UTC (permalink / raw)
  To: John Hubbard
  Cc: Kirill A. Shutemov, Andrew Morton, Al Viro, Christoph Hellwig,
	Dan Williams, Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML

On Wed, Jan 29, 2020 at 02:26:06PM -0800, John Hubbard wrote:
> On 1/29/20 3:25 AM, Kirill A. Shutemov wrote:
> > On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
> >> When debugging a problem that involves compound pages, it is extremely
> >> helpful if dump_page() reports not only the page->_refcount, but also
> >> the refcount of the head page of the compound page. That's because the
> >> head page collects refcounts for the entire compound page.
> >>
> >> Therefore, enhance dump_page() so as to print out the refcount of the
> >> head page of a compound page.
> >>
> >> This approach (printing information about a struct page that is not the
> >> struct page that was passed into dump_page()) has a precedent:
> >> compound_mapcount is already being printed.
> > 
> > refcount on a tail must always be 0. I think we should only print it when
> > it is non-zero, emphasizing this fact with a standalone message.
> > 
> 
> Hi Kirill,
> 
> Yes, good point, that sounds like just the right balance. And it avoids adding 
> a new item to print in the common case, which is very nice. Here's what I've
> changed it to for the next version (I'll also rewrite the commit description 
> accordingly):
> 
> 
> diff --git a/mm/debug.c b/mm/debug.c
> index a90da5337c14..3a45e2b77de0 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -75,12 +75,17 @@ void __dump_page(struct page *page, const char *reason)
>  	 */
>  	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
>  
> -	if (PageCompound(page))
> -		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
> -			"index:%#lx compound_mapcount: %d\n",
> -			page, page_ref_count(page), mapcount,
> +	if (PageCompound(page)) {
> +		pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
> +			"index:%#lx compound_mapcount:%d\n",
> +			page, page_ref_count(compound_head(page)), mapcount,
>  			page->mapping, page_to_pgoff(page),
>  			compound_mapcount(page));
> +
> +		if (page != compound_head(page) && page_ref_count(page) != 0)
> +			pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
> +				"this tail page\n", page, page_ref_count(page));
> +	}
>  	else
>  		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
>  			page, page_ref_count(page), mapcount,

I have a hunk in my current tree which looks like this:

@@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason)
                pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx
\n",
                        page, page_ref_count(page), mapcount,
                        page->mapping, page_to_pgoff(page));
+       if (PageTail(page)) {
+               struct page *head = compound_head(page);
+               pr_warn("head:%px mapping:%px index:%#lx\n",
+                       head, head->mapping, page_to_pgoff(head));
+       }
        if (PageKsm(page))
                pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags);
        else if (PageAnon(page))

I wonder if we can combine these two patches in some more useful way?

I also think we probably want a sanity check that 'head' and 'page'
are within a sane range of each other (ie head < page and head +
MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains
complete garbage.

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
  2020-01-29 22:59       ` Matthew Wilcox
@ 2020-01-30  6:23         ` John Hubbard
  2020-01-30  6:30           ` John Hubbard
  0 siblings, 1 reply; 18+ messages in thread
From: John Hubbard @ 2020-01-30  6:23 UTC (permalink / raw)
  To: Matthew Wilcox
  Cc: Kirill A. Shutemov, Andrew Morton, Al Viro, Christoph Hellwig,
	Dan Williams, Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML

On 1/29/20 2:59 PM, Matthew Wilcox wrote:
...
> I have a hunk in my current tree which looks like this:
> 
> @@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason)
>                  pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx
> \n",
>                          page, page_ref_count(page), mapcount,
>                          page->mapping, page_to_pgoff(page));
> +       if (PageTail(page)) {
> +               struct page *head = compound_head(page);
> +               pr_warn("head:%px mapping:%px index:%#lx\n",
> +                       head, head->mapping, page_to_pgoff(head));
> +       }
>          if (PageKsm(page))
>                  pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags);
>          else if (PageAnon(page))
> 
> I wonder if we can combine these two patches in some more useful way?
> 
> I also think we probably want a sanity check that 'head' and 'page'
> are within a sane range of each other (ie head < page and head +
> MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains
> complete garbage.
> 

OK, here's a go at combining those. I like the observation, implicit in your
diffs, that PageTail rather than PageCompound is the key differentiator in
deciding what to print. How's this look:

diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..944652843e7b 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -75,12 +75,31 @@ void __dump_page(struct page *page, const char *reason)
  	 */
  	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
  
-	if (PageCompound(page))
-		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
-			"index:%#lx compound_mapcount: %d\n",
-			page, page_ref_count(page), mapcount,
-			page->mapping, page_to_pgoff(page),
-			compound_mapcount(page));
+	if (PageTail(page)) {
+		struct page *head = compound_head(page);
+
+		if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) {
+			/*
+			 * Page is hopelessly corrupted, so limit any reporting
+			 * to information about the page itself. Do not attempt
+			 * to look at the head page.
+			 */
+			pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
+				"index:%#lx (corrupted tail page case)\n",
+				page, page_ref_count(page), mapcount,
+				page->mapping, page_to_pgoff(page));
+		} else {
+			pr_warn("page:%px compound refcount:%d mapcount:%d "
+				"mapping:%px index:%#lx compound_mapcount:%d\n",
+				page, page_ref_count(head),
+				mapcount, head->mapping, page_to_pgoff(head),
+				compound_mapcount(page));
+
+			if (page_ref_count(page) != 0)
+				pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
+					"this tail page\n", page, page_ref_count(page));
+		}
+	}
  	else
  		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
  			page, page_ref_count(page), mapcount,

?

Here's sample output for a normal page, a tail page, and a tail page with a bad
(non-zero) refcount:

============
Normal page:
============
[   38.572084] page:ffffea0011465880 refcount:2 mapcount:1 mapping:ffff888454d99001 index:0xb2
[   38.579256] anon flags: 0x17ffe0000080036(referenced|uptodate|lru|active|swapbacked)
[   38.585799] raw: 017ffe0000080036 ffffea0011460fc8 ffffea0011466d08 ffff888454d99001
[   38.592350] raw: 00000000000000b2 0000000000000000 0000000200000000 0000000000000000
[   38.598885] page dumped because: test dump page


==========
Tail page:
==========
[   38.436384] page:ffffea0010aa0280 compound refcount:503 mapcount:1 mapping:ffff888455fb3399 index:0xa8 compound_mapcount:1
[   38.446350] anon flags: 0x17ffe0000000000()
[   38.449661] raw: 017ffe0000000000 ffffea0010aa0001 ffffea0010aa0288 dead000000000400
[   38.456228] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[   38.462794] page dumped because: test dump page

============================
Tail page with bad refcount:
============================
[   38.466088] page:ffffea0010aa0b40 compound refcount:468 mapcount:1 mapping:ffff888455fb3399 index:0xa8 compound_mapcount:1
[   38.475967] page:ffffea0010aa0b40 PROBLEM: non-zero refcount (==2) on this tail page
[   38.482490] anon flags: 0x17ffe0000000000()
[   38.485432] raw: 017ffe0000000000 ffffea0010aa0001 ffffea0010aa0b48 dead000000000400
[   38.491996] raw: 0000000000000000 0000000000000000 00000002ffffffff 0000000000000000
[   38.498532] page dumped because: test bad tail page refcount



thanks,
-- 
John Hubbard
NVIDIA

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
  2020-01-30  6:23         ` John Hubbard
@ 2020-01-30  6:30           ` John Hubbard
  0 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-30  6:30 UTC (permalink / raw)
  To: Matthew Wilcox
  Cc: Kirill A. Shutemov, Andrew Morton, Al Viro, Christoph Hellwig,
	Dan Williams, Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML

On 1/29/20 10:23 PM, John Hubbard wrote:
> On 1/29/20 2:59 PM, Matthew Wilcox wrote:
> ...
>> I have a hunk in my current tree which looks like this:
>>
>> @@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason)
>>                  pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx
>> \n",
>>                          page, page_ref_count(page), mapcount,
>>                          page->mapping, page_to_pgoff(page));
>> +       if (PageTail(page)) {
>> +               struct page *head = compound_head(page);
>> +               pr_warn("head:%px mapping:%px index:%#lx\n",
>> +                       head, head->mapping, page_to_pgoff(head));
>> +       }
>>          if (PageKsm(page))
>>                  pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags);
>>          else if (PageAnon(page))
>>
>> I wonder if we can combine these two patches in some more useful way?
>>
>> I also think we probably want a sanity check that 'head' and 'page'
>> are within a sane range of each other (ie head < page and head +
>> MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains
>> complete garbage.
>>
> 
> OK, here's a go at combining those. I like the observation, implicit in your
> diffs, that PageTail rather than PageCompound is the key differentiator in
> deciding what to print. How's this look:
> 
> diff --git a/mm/debug.c b/mm/debug.c
> index a90da5337c14..944652843e7b 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -75,12 +75,31 @@ void __dump_page(struct page *page, const char *reason)
>        */
>       mapcount = PageSlab(page) ? 0 : page_mapcount(page);
> 
> -    if (PageCompound(page))
> -        pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
> -            "index:%#lx compound_mapcount: %d\n",
> -            page, page_ref_count(page), mapcount,
> -            page->mapping, page_to_pgoff(page),
> -            compound_mapcount(page));
> +    if (PageTail(page)) {
> +        struct page *head = compound_head(page);
> +
> +        if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) {
> +            /*
> +             * Page is hopelessly corrupted, so limit any reporting
> +             * to information about the page itself. Do not attempt
> +             * to look at the head page.
> +             */
> +            pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
> +                "index:%#lx (corrupted tail page case)\n",
> +                page, page_ref_count(page), mapcount,
> +                page->mapping, page_to_pgoff(page));
> +        } else {
> +            pr_warn("page:%px compound refcount:%d mapcount:%d "
> +                "mapping:%px index:%#lx compound_mapcount:%d\n",
> +                page, page_ref_count(head),
> +                mapcount, head->mapping, page_to_pgoff(head),
> +                compound_mapcount(page));
> +
> +            if (page_ref_count(page) != 0)
> +                pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
> +                    "this tail page\n", page, page_ref_count(page));

...ahem, I sorta botched the above statement, because that should
be outside (just below) the "else" statement--it can be done whether or
not the page fails the safety/bounds check. :)

thanks,
-- 
John Hubbard
NVIDIA

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages
  2020-01-29 13:51   ` Kirill A. Shutemov
@ 2020-01-30  6:44     ` John Hubbard
  2020-01-30 11:31       ` Kirill A. Shutemov
  0 siblings, 1 reply; 18+ messages in thread
From: John Hubbard @ 2020-01-30  6:44 UTC (permalink / raw)
  To: Kirill A. Shutemov
  Cc: Andrew Morton, Al Viro, Christoph Hellwig, Dan Williams,
	Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	Kirill A . Shutemov

On 1/29/20 5:51 AM, Kirill A. Shutemov wrote:
> On Tue, Jan 28, 2020 at 07:24:13PM -0800, John Hubbard wrote:
>> Add tracking of pages that were pinned via FOLL_PIN. This tracking is
>> implemented via overloading of page->_refcount: pins are added by
>> adding GUP_PIN_COUNTING_BIAS (1024) to the refcount. This provides a
>> fuzzy indication of pinning, and it can have false positives (and that's
>> OK). Please see the pre-existing
>> Documentation/core-api/pin_user_pages.rst for details.
>>
>> As mentioned in pin_user_pages.rst, callers who effectively set FOLL_PIN
>> (typically via pin_user_pages*()) are required to ultimately free such
>> pages via unpin_user_page().
>>
>> Please also not the limitation, discussed in pin_user_pages.rst under
> 
> s/not/note/

Fixed, thanks!

...
>>   
>> +/**
>> + * page_dma_pinned() - report if a page is pinned for DMA.
>> + *
>> + * This function checks if a page has been pinned via a call to
>> + * pin_user_pages*().
>> + *
>> + * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
>> + * because it means "definitely not pinned for DMA", but true means "probably
>> + * pinned for DMA, but possibly a false positive due to having at least
>> + * GUP_PIN_COUNTING_BIAS worth of normal page references".
>> + *
>> + * False positives are OK, because: a) it's unlikely for a page to get that many
>> + * refcounts, and b) all the callers of this routine are expected to be able to
>> + * deal gracefully with a false positive.
> 
> I wounder if we should reverse the logic and name -- page_not_dma_pinned()
> or something -- too emphasise that we can only know for sure when the page
> is not pinned, but not necessary when it is.
> 

This is an interesting point. I agree that it's worth maybe adding information
into the function name, but I'd like to keep the bool "positive", because there
will be a number of callers that ask "if it is possibly dma-pinned, then ...".
So combining that, how about this function name:

	page_maybe_dma_pinned()

, which I could live with and I think would be acceptable?

>> + *
>> + * For more information, please see Documentation/vm/pin_user_pages.rst.
>> + *
>> + * @page:	pointer to page to be queried.
>> + * @Return:	True, if it is likely that the page has been "dma-pinned".
>> + *		False, if the page is definitely not dma-pinned.
>> + */
>> +static inline bool page_dma_pinned(struct page *page)
>> +{
>> +	/*
>> +	 * page_ref_count() is signed. If that refcount overflows, then
>> +	 * page_ref_count() returns a negative value, and callers will avoid
>> +	 * further incrementing the refcount.
>> +	 *
>> +	 * Here, for that overflow case, use the signed bit to count a little
>> +	 * bit higher via unsigned math, and thus still get an accurate result
>> +	 * from page_dma_pinned().
>> +	 */
>> +	return ((unsigned int)page_ref_count(compound_head(page))) >=
>> +		GUP_PIN_COUNTING_BIAS;
> 
> Do you expect it too be called on tail pages?


Yes, we definitely cannot rule that out.


> 
>> +}
>> +
>>   #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>>   #define SECTION_IN_PAGE_FLAGS
>>   #endif
>> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
>> index 14d14beb1f7f..b9cbe553d1e7 100644
>> --- a/include/linux/page_ref.h
>> +++ b/include/linux/page_ref.h
>> @@ -102,6 +102,16 @@ static inline void page_ref_sub(struct page *page, int nr)
>>   		__page_ref_mod(page, -nr);
>>   }
>>   
>> +static inline int page_ref_sub_return(struct page *page, int nr)
>> +{
>> +	int ret = atomic_sub_return(nr, &page->_refcount);
>> +
>> +	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
>> +		__page_ref_mod(page, -nr);
>> +
>> +	return ret;
>> +}
>> +
> 
> I see opportunity to split the patch further.


ah, OK. I wasn't sure how far to go before I get tagged for "excessive
patch splitting"! haha. Anyway, are you suggesting to put the
page_ref_sub_return() routine into it's own patch?

Another thing to split out would be adding the flags to the remaining
functions, such as undo_dev_pagemap(). That burns quite a few lines of
diff. Anything else to split out?

> 
>>   static inline void page_ref_inc(struct page *page)
>>   {
>>   	atomic_inc(&page->_refcount);
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 9e117998274c..7a96490dcc54 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -44,6 +44,136 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
>>   	return head;
>>   }
>>   
>> +/*
>> + * try_grab_compound_head() - attempt to elevate a page's refcount, by a
>> + * flags-dependent amount.
>> + *
>> + * "grab" names in this file mean, "look at flags to decide whether to use
>> + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
>> + *
>> + * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
>> + * same time. (That's true throughout the get_user_pages*() and
>> + * pin_user_pages*() APIs.) Cases:
>> + *
>> + *    FOLL_GET: page's refcount will be incremented by 1.
>> + *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
>> + *
>> + * Return: head page (with refcount appropriately incremented) for success, or
>> + * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
>> + * considered failure, and furthermore, a likely bug in the caller, so a warning
>> + * is also emitted.
>> + */
>> +static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>> +							  int refs,
>> +							  unsigned int flags)
>> +{
>> +	if (flags & FOLL_GET)
>> +		return try_get_compound_head(page, refs);
>> +	else if (flags & FOLL_PIN) {
>> +		refs *= GUP_PIN_COUNTING_BIAS;
>> +		return try_get_compound_head(page, refs);
> 
> Maybe overflow detection? At least under VM_BUG_ON()?


OK, yes I see now that there is no check to see if we're about to overflow
the refs, in this path. I'll add one.


...
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 0a55dec68925..b1079aaa6f24 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   	 */
>>   	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
>>   
>> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
>> +			 (FOLL_PIN | FOLL_GET)))
> 
> Too many parentheses.


OK, I'll remove at least one. :)


> 
>> +		return NULL;
>> +
>>   	if (flags & FOLL_WRITE && !pmd_write(*pmd))
>>   		return NULL;
>>   
>> @@ -973,7 +978,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   	 * device mapped pages can only be returned if the
>>   	 * caller will manage the page reference count.
>>   	 */
>> -	if (!(flags & FOLL_GET))
>> +	if (!(flags & (FOLL_GET | FOLL_PIN)))
>>   		return ERR_PTR(-EEXIST);
>>   
>>   	pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
>> @@ -981,7 +986,8 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   	if (!*pgmap)
>>   		return ERR_PTR(-EFAULT);
>>   	page = pfn_to_page(pfn);
>> -	get_page(page);
>> +	if (!try_grab_page(page, flags))
>> +		page = ERR_PTR(-ENOMEM);
>>   
>>   	return page;
>>   }
>> @@ -1101,6 +1107,11 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
>>   	if (flags & FOLL_WRITE && !pud_write(*pud))
>>   		return NULL;
>>   
>> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
>> +			 (FOLL_PIN | FOLL_GET)))
>> +		return NULL;
>> +
> 
> Ditto.


ACK.

...
>> @@ -4965,6 +4958,12 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>>   	struct page *page = NULL;
>>   	spinlock_t *ptl;
>>   	pte_t pte;
>> +
>> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
>> +			 (FOLL_PIN | FOLL_GET)))
>> +		return NULL;
>> +
> 
> Ditto.

ACK.




thanks,
-- 
John Hubbard
NVIDIA

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages
  2020-01-30  6:44     ` John Hubbard
@ 2020-01-30 11:31       ` Kirill A. Shutemov
  2020-01-31  3:19         ` John Hubbard
  0 siblings, 1 reply; 18+ messages in thread
From: Kirill A. Shutemov @ 2020-01-30 11:31 UTC (permalink / raw)
  To: John Hubbard
  Cc: Andrew Morton, Al Viro, Christoph Hellwig, Dan Williams,
	Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	Kirill A . Shutemov

On Wed, Jan 29, 2020 at 10:44:50PM -0800, John Hubbard wrote:
> On 1/29/20 5:51 AM, Kirill A. Shutemov wrote:
> > > +/**
> > > + * page_dma_pinned() - report if a page is pinned for DMA.
> > > + *
> > > + * This function checks if a page has been pinned via a call to
> > > + * pin_user_pages*().
> > > + *
> > > + * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
> > > + * because it means "definitely not pinned for DMA", but true means "probably
> > > + * pinned for DMA, but possibly a false positive due to having at least
> > > + * GUP_PIN_COUNTING_BIAS worth of normal page references".
> > > + *
> > > + * False positives are OK, because: a) it's unlikely for a page to get that many
> > > + * refcounts, and b) all the callers of this routine are expected to be able to
> > > + * deal gracefully with a false positive.
> > 
> > I wounder if we should reverse the logic and name -- page_not_dma_pinned()
> > or something -- too emphasise that we can only know for sure when the page
> > is not pinned, but not necessary when it is.
> > 
> 
> This is an interesting point. I agree that it's worth maybe adding information
> into the function name, but I'd like to keep the bool "positive", because there
> will be a number of callers that ask "if it is possibly dma-pinned, then ...".
> So combining that, how about this function name:
> 
> 	page_maybe_dma_pinned()
> 
> , which I could live with and I think would be acceptable?

I would still prefer the negative version, but up to you.

> > I see opportunity to split the patch further.
> 
> 
> ah, OK. I wasn't sure how far to go before I get tagged for "excessive
> patch splitting"! haha. Anyway, are you suggesting to put the
> page_ref_sub_return() routine into it's own patch?
> 
> Another thing to split out would be adding the flags to the remaining
> functions, such as undo_dev_pagemap(). That burns quite a few lines of
> diff. Anything else to split out?

Nothing I see immediately.

> 
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 0a55dec68925..b1079aaa6f24 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > >   	 */
> > >   	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
> > > +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > > +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> > > +			 (FOLL_PIN | FOLL_GET)))
> > 
> > Too many parentheses.
> 
> 
> OK, I'll remove at least one. :)

I see two.

-- 
 Kirill A. Shutemov

^ permalink raw reply	[flat|nested] 18+ messages in thread

* Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages
  2020-01-30 11:31       ` Kirill A. Shutemov
@ 2020-01-31  3:19         ` John Hubbard
  0 siblings, 0 replies; 18+ messages in thread
From: John Hubbard @ 2020-01-31  3:19 UTC (permalink / raw)
  To: Kirill A. Shutemov
  Cc: Andrew Morton, Al Viro, Christoph Hellwig, Dan Williams,
	Dave Chinner, Ira Weiny, Jan Kara, Jason Gunthorpe,
	Jonathan Corbet, Jérôme Glisse, Michal Hocko,
	Mike Kravetz, Shuah Khan, Vlastimil Babka, linux-doc,
	linux-fsdevel, linux-kselftest, linux-rdma, linux-mm, LKML,
	Kirill A . Shutemov

On 1/30/20 3:31 AM, Kirill A. Shutemov wrote:
...
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 0a55dec68925..b1079aaa6f24 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>>>>   	 */
>>>>   	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
>>>> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>>>> +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
>>>> +			 (FOLL_PIN | FOLL_GET)))
>>>
>>> Too many parentheses.
>>
>>
>> OK, I'll remove at least one. :)
> 
> I see two.

ah, correction: actually, the original statement has exactly the right number of parentheses.
The relevant C precedence order is:
	==
	&
	|

...which means that both "&" and "|" operations need parentheses protection from the higher
precedence "==" operation.

(There are other places in the kernel that have this exact pattern, too, with the same
pattern of parentheses that I'm using, of course.)

thanks,
-- 
John Hubbard
NVIDIA
 

^ permalink raw reply	[flat|nested] 18+ messages in thread

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Thread overview: 18+ messages (download: mbox.gz / follow: Atom feed)
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2020-01-29  3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
2020-01-29  3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
2020-01-29 11:25   ` Kirill A. Shutemov
2020-01-29 22:26     ` John Hubbard
2020-01-29 22:59       ` Matthew Wilcox
2020-01-30  6:23         ` John Hubbard
2020-01-30  6:30           ` John Hubbard
2020-01-29  3:24 ` [PATCH v2 2/8] mm/gup: split get_user_pages_remote() into two routines John Hubbard
2020-01-29  3:24 ` [PATCH v2 3/8] mm/gup: pass a flags arg to __gup_device_* functions John Hubbard
2020-01-29  3:24 ` [PATCH v2 4/8] mm/gup: track FOLL_PIN pages John Hubbard
2020-01-29 13:51   ` Kirill A. Shutemov
2020-01-30  6:44     ` John Hubbard
2020-01-30 11:31       ` Kirill A. Shutemov
2020-01-31  3:19         ` John Hubbard
2020-01-29  3:24 ` [PATCH v2 5/8] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages John Hubbard
2020-01-29  3:24 ` [PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting John Hubbard
2020-01-29  3:24 ` [PATCH v2 7/8] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2020-01-29  3:24 ` [PATCH v2 8/8] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard

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