[PATCH v2 0/4] mm/gup, drm/i915: refactor gup_fast, convert to pin_user_pages()

[PATCH v2 0/4] mm/gup, drm/i915: refactor gup_fast, convert to pin_user_pages()

[John Hubbard]

The purpose of posting this series is to launch a test in the
intel-gfx-ci tree. (The patches have already been merged into Andrew's
linux-mm tree.)

This applies to today's linux.git (note the base-commit tag at the
bottom).

Changes since V1:

* Fixed a bug in the refactoring patch: added FOLL_FAST_ONLY to the
  list of gup_flags *not* to WARN() on. This lead to a failure in the
  first intel-gfx-ci test run [1].

[1] https://lore.kernel.org/r/159008745422.32320.5724805750977048669@build.alporthouse.com

Original cover letter:

This needs to go through Andrew's -mm tree, due to adding a new gup.c
routine. However, I would really love to have some testing from the
drm/i915 folks, because I haven't been able to run-time test that part
of it.

Otherwise, though, the series has passed my basic run time testing:
some LTP tests, some xfs and etx4 non-destructive xfstests, and an
assortment of other smaller ones: vm selftests, io_uring_register, a
few more. But that's only on one particular machine. Also, cross-compile
tests for half a dozen arches all pass.

Details:

In order to convert the drm/i915 driver from get_user_pages() to
pin_user_pages(), a FOLL_PIN equivalent of __get_user_pages_fast() was
required. That led to refactoring __get_user_pages_fast(), with the
following goals:

1) As above: provide a pin_user_pages*() routine for drm/i915 to call,
   in place of __get_user_pages_fast(),

2) Get rid of the gup.c duplicate code for walking page tables with
   interrupts disabled. This duplicate code is a minor maintenance
   problem anyway.

3) Make it easy for an upcoming patch from Souptick, which aims to
   convert __get_user_pages_fast() to use a gup_flags argument, instead
   of a bool writeable arg.  Also, if this series looks good, we can
   ask Souptick to change the name as well, to whatever the consensus
   is. My initial recommendation is: get_user_pages_fast_only(), to
   match the new pin_user_pages_only().

John Hubbard (4):
  mm/gup: move __get_user_pages_fast() down a few lines in gup.c
  mm/gup: refactor and de-duplicate gup_fast() code
  mm/gup: introduce pin_user_pages_fast_only()
  drm/i915: convert get_user_pages() --> pin_user_pages()

 drivers/gpu/drm/i915/gem/i915_gem_userptr.c |  22 +--
 include/linux/mm.h                          |   3 +
 mm/gup.c                                    | 153 ++++++++++++--------
 3 files changed, 109 insertions(+), 69 deletions(-)


base-commit: 051143e1602d90ea71887d92363edd539d411de5
-- 
2.26.2

[PATCH v2 1/4] mm/gup: move __get_user_pages_fast() down a few lines in gup.c

[John Hubbard]

This is in order to avoid a forward declaration of
internal_get_user_pages_fast(), in the next patch.

This is code movement only--all generated code should
be identical.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 112 +++++++++++++++++++++++++++----------------------------
 1 file changed, 56 insertions(+), 56 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 50cd9323efff..4502846d57f9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2666,62 +2666,6 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
 }
 #endif
 
-/*
- * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
- * the regular GUP.
- * Note a difference with get_user_pages_fast: this always returns the
- * number of pages pinned, 0 if no pages were pinned.
- *
- * If the architecture does not support this function, simply return with no
- * pages pinned.
- */
-int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
-			  struct page **pages)
-{
-	unsigned long len, end;
-	unsigned long flags;
-	int nr_pinned = 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;
-	end = start + len;
-
-	if (end <= start)
-		return 0;
-	if (unlikely(!access_ok((void __user *)start, len)))
-		return 0;
-
-	/*
-	 * Disable interrupts.  We use the nested form as we can already have
-	 * interrupts disabled by get_futex_key.
-	 *
-	 * With interrupts disabled, we block page table pages from being
-	 * freed from under us. See struct mmu_table_batch comments in
-	 * include/asm-generic/tlb.h for more details.
-	 *
-	 * We do not adopt an rcu_read_lock(.) here as we also want to
-	 * block IPIs that come from THPs splitting.
-	 */
-
-	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
-	    gup_fast_permitted(start, end)) {
-		local_irq_save(flags);
-		gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
-		local_irq_restore(flags);
-	}
-
-	return nr_pinned;
-}
-EXPORT_SYMBOL_GPL(__get_user_pages_fast);
-
 static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
 				   unsigned int gup_flags, struct page **pages)
 {
@@ -2794,6 +2738,62 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 	return ret;
 }
 
+/*
+ * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
+ * the regular GUP.
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
+ *
+ * If the architecture does not support this function, simply return with no
+ * pages pinned.
+ */
+int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+			  struct page **pages)
+{
+	unsigned long len, end;
+	unsigned long flags;
+	int nr_pinned = 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;
+	end = start + len;
+
+	if (end <= start)
+		return 0;
+	if (unlikely(!access_ok((void __user *)start, len)))
+		return 0;
+
+	/*
+	 * Disable interrupts.  We use the nested form as we can already have
+	 * interrupts disabled by get_futex_key.
+	 *
+	 * With interrupts disabled, we block page table pages from being
+	 * freed from under us. See struct mmu_table_batch comments in
+	 * include/asm-generic/tlb.h for more details.
+	 *
+	 * We do not adopt an rcu_read_lock(.) here as we also want to
+	 * block IPIs that come from THPs splitting.
+	 */
+
+	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
+	    gup_fast_permitted(start, end)) {
+		local_irq_save(flags);
+		gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
+		local_irq_restore(flags);
+	}
+
+	return nr_pinned;
+}
+EXPORT_SYMBOL_GPL(__get_user_pages_fast);
+
 /**
  * get_user_pages_fast() - pin user pages in memory
  * @start:      starting user address
-- 
2.26.2

[PATCH v2 2/4] mm/gup: refactor and de-duplicate gup_fast() code

[John Hubbard]

There were two nearly identical sets of code for gup_fast()
style of walking the page tables with interrupts disabled.
This has lead to the usual maintenance problems that arise from
having duplicated code.

There is already a core internal routine in gup.c for gup_fast(),
so just enhance it very slightly: allow skipping the fall-back
to "slow" (regular) get_user_pages(), via the new FOLL_FAST_ONLY
flag. Then, just call internal_get_user_pages_fast() from
__get_user_pages_fast(), and adjust the API to match pre-existing
API behavior.

There is a change in behavior from this refactoring: the nested
form of interrupt disabling is used in all gup_fast() variants
now. That's because there is only one place that interrupt disabling
for page walking is done, and so the safer form is required. This
should, if anything, eliminate possible (rare) bugs, because the
non-nested form of enabling interrupts was fragile at best.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h |  1 +
 mm/gup.c           | 63 ++++++++++++++++++++++------------------------
 2 files changed, 31 insertions(+), 33 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a5594ac9ebe3..84b601cab699 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2782,6 +2782,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_LONGTERM	0x10000	/* mapping lifetime is indefinite: see below */
 #define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
 #define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
+#define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
 
 /*
  * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
diff --git a/mm/gup.c b/mm/gup.c
index 4502846d57f9..4564b0dc7d0b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2694,10 +2694,12 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 					struct page **pages)
 {
 	unsigned long addr, len, end;
+	unsigned long flags;
 	int nr_pinned = 0, ret = 0;
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
-				       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
+				       FOLL_FORCE | FOLL_PIN | FOLL_GET |
+				       FOLL_FAST_ONLY)))
 		return -EINVAL;
 
 	start = untagged_addr(start) & PAGE_MASK;
@@ -2710,15 +2712,26 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 	if (unlikely(!access_ok((void __user *)start, len)))
 		return -EFAULT;
 
+	/*
+	 * Disable interrupts. The nested form is used, in order to allow full,
+	 * general purpose use of this routine.
+	 *
+	 * With interrupts disabled, we block page table pages from being
+	 * freed from under us. See struct mmu_table_batch comments in
+	 * include/asm-generic/tlb.h for more details.
+	 *
+	 * We do not adopt an rcu_read_lock(.) here as we also want to
+	 * block IPIs that come from THPs splitting.
+	 */
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
-		local_irq_disable();
+		local_irq_save(flags);
 		gup_pgd_range(addr, end, gup_flags, pages, &nr_pinned);
-		local_irq_enable();
+		local_irq_restore(flags);
 		ret = nr_pinned;
 	}
 
-	if (nr_pinned < nr_pages) {
+	if (nr_pinned < nr_pages && !(gup_flags & FOLL_FAST_ONLY)) {
 		/* Try to get the remaining pages with get_user_pages */
 		start += nr_pinned << PAGE_SHIFT;
 		pages += nr_pinned;
@@ -2750,45 +2763,29 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
 {
-	unsigned long len, end;
-	unsigned long flags;
-	int nr_pinned = 0;
+	int nr_pinned;
 	/*
 	 * 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.
+	 *
+	 * FOLL_FAST_ONLY is required in order to match the API description of
+	 * this routine: no fall back to regular ("slow") GUP.
 	 */
-	unsigned int gup_flags = FOLL_GET;
+	unsigned int gup_flags = FOLL_GET | FOLL_FAST_ONLY;
 
 	if (write)
 		gup_flags |= FOLL_WRITE;
 
-	start = untagged_addr(start) & PAGE_MASK;
-	len = (unsigned long) nr_pages << PAGE_SHIFT;
-	end = start + len;
-
-	if (end <= start)
-		return 0;
-	if (unlikely(!access_ok((void __user *)start, len)))
-		return 0;
-
+	nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
+						 pages);
 	/*
-	 * Disable interrupts.  We use the nested form as we can already have
-	 * interrupts disabled by get_futex_key.
-	 *
-	 * With interrupts disabled, we block page table pages from being
-	 * freed from under us. See struct mmu_table_batch comments in
-	 * include/asm-generic/tlb.h for more details.
-	 *
-	 * We do not adopt an rcu_read_lock(.) here as we also want to
-	 * block IPIs that come from THPs splitting.
+	 * As specified in the API description above, this routine is not
+	 * allowed to return negative values. However, the common core
+	 * routine internal_get_user_pages_fast() *can* return -errno.
+	 * Therefore, correct for that here:
 	 */
-
-	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
-	    gup_fast_permitted(start, end)) {
-		local_irq_save(flags);
-		gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
-		local_irq_restore(flags);
-	}
+	if (nr_pinned < 0)
+		nr_pinned = 0;
 
 	return nr_pinned;
 }
-- 
2.26.2

[PATCH v2 3/4] mm/gup: introduce pin_user_pages_fast_only()

[John Hubbard]

This is the FOLL_PIN equivalent of __get_user_pages_fast(),
except with a more descriptive name, and gup_flags instead of
a boolean "write" in the argument list.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 84b601cab699..98be7289d7e9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1820,6 +1820,8 @@ extern int mprotect_fixup(struct vm_area_struct *vma,
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages);
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages);
 /*
  * per-process(per-mm_struct) statistics.
  */
diff --git a/mm/gup.c b/mm/gup.c
index 4564b0dc7d0b..6fa9b2016a53 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2859,6 +2859,42 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
 }
 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
 
+/*
+ * This is the FOLL_PIN equivalent of __get_user_pages_fast(). Behavior is the
+ * same, except that this one sets FOLL_PIN instead of FOLL_GET.
+ *
+ * The API rules are the same, too: no negative values may be returned.
+ */
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages)
+{
+	int nr_pinned;
+
+	/*
+	 * FOLL_GET and FOLL_PIN are mutually exclusive. Note that the API
+	 * rules require returning 0, rather than -errno:
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return 0;
+	/*
+	 * FOLL_FAST_ONLY is required in order to match the API description of
+	 * this routine: no fall back to regular ("slow") GUP.
+	 */
+	gup_flags |= (FOLL_PIN | FOLL_FAST_ONLY);
+	nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
+						 pages);
+	/*
+	 * This routine is not allowed to return negative values. However,
+	 * internal_get_user_pages_fast() *can* return -errno. Therefore,
+	 * correct for that here:
+	 */
+	if (nr_pinned < 0)
+		nr_pinned = 0;
+
+	return nr_pinned;
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);
+
 /**
  * pin_user_pages_remote() - pin pages of a remote process (task != current)
  *
-- 
2.26.2

[PATCH v2 4/4] drm/i915: convert get_user_pages() --> pin_user_pages()

[John Hubbard]

This code was using get_user_pages*(), in a "Case 2" scenario
(DMA/RDMA), using the categorization from [1]. That means that it's
time to convert the get_user_pages*() + put_page() calls to
pin_user_pages*() + unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 22 ++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
index 7ffd7afeb7a5..b55ac7563189 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -471,7 +471,7 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 					down_read(&mm->mmap_sem);
 					locked = 1;
 				}
-				ret = get_user_pages_remote
+				ret = pin_user_pages_remote
 					(work->task, mm,
 					 obj->userptr.ptr + pinned * PAGE_SIZE,
 					 npages - pinned,
@@ -507,7 +507,7 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 	}
 	mutex_unlock(&obj->mm.lock);
 
-	release_pages(pvec, pinned);
+	unpin_user_pages(pvec, pinned);
 	kvfree(pvec);
 
 	i915_gem_object_put(obj);
@@ -564,6 +564,7 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 	struct sg_table *pages;
 	bool active;
 	int pinned;
+	unsigned int gup_flags = 0;
 
 	/* If userspace should engineer that these pages are replaced in
 	 * the vma between us binding this page into the GTT and completion
@@ -598,11 +599,14 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 				      GFP_KERNEL |
 				      __GFP_NORETRY |
 				      __GFP_NOWARN);
-		if (pvec) /* defer to worker if malloc fails */
-			pinned = __get_user_pages_fast(obj->userptr.ptr,
-						       num_pages,
-						       !i915_gem_object_is_readonly(obj),
-						       pvec);
+		/* defer to worker if malloc fails */
+		if (pvec) {
+			if (!i915_gem_object_is_readonly(obj))
+				gup_flags |= FOLL_WRITE;
+			pinned = pin_user_pages_fast_only(obj->userptr.ptr,
+							  num_pages, gup_flags,
+							  pvec);
+		}
 	}
 
 	active = false;
@@ -620,7 +624,7 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 		__i915_gem_userptr_set_active(obj, true);
 
 	if (IS_ERR(pages))
-		release_pages(pvec, pinned);
+		unpin_user_pages(pvec, pinned);
 	kvfree(pvec);
 
 	return PTR_ERR_OR_ZERO(pages);
@@ -675,7 +679,7 @@ i915_gem_userptr_put_pages(struct drm_i915_gem_object *obj,
 		}
 
 		mark_page_accessed(page);
-		put_page(page);
+		unpin_user_page(page);
 	}
 	obj->mm.dirty = false;
 
-- 
2.26.2

Re: [PATCH v2 0/4] mm/gup, drm/i915: refactor gup_fast, convert to pin_user_pages()

[Chris Wilson]

Quoting John Hubbard (2020-05-22 06:19:27)
> The purpose of posting this series is to launch a test in the
> intel-gfx-ci tree. (The patches have already been merged into Andrew's
> linux-mm tree.)
> 
> This applies to today's linux.git (note the base-commit tag at the
> bottom).
> 
> Changes since V1:
> 
> * Fixed a bug in the refactoring patch: added FOLL_FAST_ONLY to the
>   list of gup_flags *not* to WARN() on. This lead to a failure in the
>   first intel-gfx-ci test run [1].
> 
> [1] https://lore.kernel.org/r/159008745422.32320.5724805750977048669@build.alporthouse.com

Ran this through our CI, warn and subsequent lockup were gone. That
lockup is worrying me now, but that doesn't seem to be an issue from
this series.

The i915 changes were simple enough, I would have computed the pin flags
just once (since the readonly bit is static, that would be interesting
if that was allowed to change mid gup :)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris

Re: [PATCH v2 0/4] mm/gup, drm/i915: refactor gup_fast, convert to pin_user_pages()

[John Hubbard]

On 2020-05-23 02:41, Chris Wilson wrote:
> Quoting John Hubbard (2020-05-22 06:19:27)
>> The purpose of posting this series is to launch a test in the
>> intel-gfx-ci tree. (The patches have already been merged into Andrew's
>> linux-mm tree.)
>>
>> This applies to today's linux.git (note the base-commit tag at the
>> bottom).
>>
>> Changes since V1:
>>
>> * Fixed a bug in the refactoring patch: added FOLL_FAST_ONLY to the
>>    list of gup_flags *not* to WARN() on. This lead to a failure in the
>>    first intel-gfx-ci test run [1].
>>
>> [1] https://lore.kernel.org/r/159008745422.32320.5724805750977048669@build.alporthouse.com
> 
> Ran this through our CI, warn and subsequent lockup were gone. That
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Yea! Thanks again for these test runs. I really don't like posting
patches that I can't run-time test, but this CI system mitigates
that pretty well.


> lockup is worrying me now, but that doesn't seem to be an issue from
> this series.


I do think it's worth following up on. And it seems like it would be
very easy to repro: just hack in a forced failure at the call site of
pin_user_pages_fast_only(), and follow the breadcrumbs.


> 
> The i915 changes were simple enough, I would have computed the pin flags
> just once (since the readonly bit is static, that would be interesting
> if that was allowed to change mid gup :)
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> -Chris
> 

Thanks for the review! And if lifting that check up higher in the call
stack is desired, I'm all in favor of that being done...in a separate
patch. :)

I'm trying to keep a very light touch when converting these call sites.

thanks,
-- 
John Hubbard
NVIDIA