From: David Hildenbrand <david@redhat.com>
To: linux-kernel@vger.kernel.org
Cc: x86@kernel.org, linux-alpha@vger.kernel.org,
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Mike Kravetz <mike.kravetz@oracle.com>,
Muchun Song <songmuchun@bytedance.com>,
Shuah Khan <shuah@kernel.org>,
Lucas Stach <l.stach@pengutronix.de>,
David Airlie <airlied@gmail.com>,
Oded Gabbay <ogabbay@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Christoph Hellwig <hch@infradead.org>,
Alex Williamson <alex.williamson@redhat.com>,
David Hildenbrand <david@redhat.com>
Subject: [PATCH mm-unstable v1 09/20] mm/gup: reliable R/O long-term pinning in COW mappings
Date: Wed, 16 Nov 2022 11:26:48 +0100 [thread overview]
Message-ID: <20221116102659.70287-10-david@redhat.com> (raw)
In-Reply-To: <20221116102659.70287-1-david@redhat.com>
We already support reliable R/O pinning of anonymous memory. However,
assume we end up pinning (R/O long-term) a pagecache page or the shared
zeropage inside a writable private ("COW") mapping. The next write access
will trigger a write-fault and replace the pinned page by an exclusive
anonymous page in the process page tables to break COW: the pinned page no
longer corresponds to the page mapped into the process' page table.
Now that FAULT_FLAG_UNSHARE can break COW on anything mapped into a
COW mapping, let's properly break COW first before R/O long-term
pinning something that's not an exclusive anon page inside a COW
mapping. FAULT_FLAG_UNSHARE will break COW and map an exclusive anon page
instead that can get pinned safely.
With this change, we can stop using FOLL_FORCE|FOLL_WRITE for reliable
R/O long-term pinning in COW mappings.
With this change, the new R/O long-term pinning tests for non-anonymous
memory succeed:
# [RUN] R/O longterm GUP pin ... with shared zeropage
ok 151 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP pin ... with memfd
ok 152 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP pin ... with tmpfile
ok 153 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP pin ... with huge zeropage
ok 154 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB)
ok 155 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB)
ok 156 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP-fast pin ... with shared zeropage
ok 157 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP-fast pin ... with memfd
ok 158 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP-fast pin ... with tmpfile
ok 159 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP-fast pin ... with huge zeropage
ok 160 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB)
ok 161 Longterm R/O pin is reliable
# [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB)
ok 162 Longterm R/O pin is reliable
Note 1: We don't care about short-term R/O-pinning, because they have
snapshot semantics: they are not supposed to observe modifications that
happen after pinning.
As one example, assume we start direct I/O to read from a page and store
page content into a file: modifications to page content after starting
direct I/O are not guaranteed to end up in the file. So even if we'd pin
the shared zeropage, the end result would be as expected -- getting zeroes
stored to the file.
Note 2: For shared mappings we'll now always fallback to the slow path to
lookup the VMA when R/O long-term pining. While that's the necessary price
we have to pay right now, it's actually not that bad in practice: most
FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with
FOLL_FORCE because they tried dealing with COW mappings correctly ...
Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE,
such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd
populate exclusive anon pages that we can pin. There was a concern that
this could affect the memlock limit of existing setups.
For example, a VM running with VFIO could run into the memlock limit and
fail to run. However, we essentially had the same behavior already in
commit 17839856fd58 ("gup: document and work around "COW can break either
way" issue") which got merged into some enterprise distros, and there were
not any such complaints. So most probably, we're fine.
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm.h | 27 ++++++++++++++++++++++++---
mm/gup.c | 10 +++++-----
mm/huge_memory.c | 2 +-
mm/hugetlb.c | 7 ++++---
4 files changed, 34 insertions(+), 12 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6bd2ee5872dd..e8cc838f42f9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3095,8 +3095,12 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
* Must be called with the (sub)page that's actually referenced via the
* page table entry, which might not necessarily be the head page for a
* PTE-mapped THP.
+ *
+ * If the vma is NULL, we're coming from the GUP-fast path and might have
+ * to fallback to the slow path just to lookup the vma.
*/
-static inline bool gup_must_unshare(unsigned int flags, struct page *page)
+static inline bool gup_must_unshare(struct vm_area_struct *vma,
+ unsigned int flags, struct page *page)
{
/*
* FOLL_WRITE is implicitly handled correctly as the page table entry
@@ -3109,8 +3113,25 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page)
* Note: PageAnon(page) is stable until the page is actually getting
* freed.
*/
- if (!PageAnon(page))
- return false;
+ if (!PageAnon(page)) {
+ /*
+ * We only care about R/O long-term pining: R/O short-term
+ * pinning does not have the semantics to observe successive
+ * changes through the process page tables.
+ */
+ if (!(flags & FOLL_LONGTERM))
+ return false;
+
+ /* We really need the vma ... */
+ if (!vma)
+ return true;
+
+ /*
+ * ... because we only care about writable private ("COW")
+ * mappings where we have to break COW early.
+ */
+ return is_cow_mapping(vma->vm_flags);
+ }
/* Paired with a memory barrier in page_try_share_anon_rmap(). */
if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
diff --git a/mm/gup.c b/mm/gup.c
index 5182abaaecde..01116699c863 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -578,7 +578,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
}
}
- if (!pte_write(pte) && gup_must_unshare(flags, page)) {
+ if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) {
page = ERR_PTR(-EMLINK);
goto out;
}
@@ -2338,7 +2338,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
goto pte_unmap;
}
- if (!pte_write(pte) && gup_must_unshare(flags, page)) {
+ if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) {
gup_put_folio(folio, 1, flags);
goto pte_unmap;
}
@@ -2506,7 +2506,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
return 0;
}
- if (!pte_write(pte) && gup_must_unshare(flags, &folio->page)) {
+ if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
return 0;
}
@@ -2572,7 +2572,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
return 0;
}
- if (!pmd_write(orig) && gup_must_unshare(flags, &folio->page)) {
+ if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
return 0;
}
@@ -2612,7 +2612,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
return 0;
}
- if (!pud_write(orig) && gup_must_unshare(flags, &folio->page)) {
+ if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
return 0;
}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 68d00196b519..dec7a7c0eca8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1434,7 +1434,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
if (pmd_protnone(*pmd) && !gup_can_follow_protnone(flags))
return NULL;
- if (!pmd_write(*pmd) && gup_must_unshare(flags, page))
+ if (!pmd_write(*pmd) && gup_must_unshare(vma, flags, page))
return ERR_PTR(-EMLINK);
VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 383b26069b33..c3aab6d5b7aa 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6195,7 +6195,8 @@ static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
}
}
-static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
+static inline bool __follow_hugetlb_must_fault(struct vm_area_struct *vma,
+ unsigned int flags, pte_t *pte,
bool *unshare)
{
pte_t pteval = huge_ptep_get(pte);
@@ -6207,7 +6208,7 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
return false;
if (flags & FOLL_WRITE)
return true;
- if (gup_must_unshare(flags, pte_page(pteval))) {
+ if (gup_must_unshare(vma, flags, pte_page(pteval))) {
*unshare = true;
return true;
}
@@ -6336,7 +6337,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
* directly from any kind of swap entries.
*/
if (absent ||
- __follow_hugetlb_must_fault(flags, pte, &unshare)) {
+ __follow_hugetlb_must_fault(vma, flags, pte, &unshare)) {
vm_fault_t ret;
unsigned int fault_flags = 0;
--
2.38.1
next prev parent reply other threads:[~2022-11-16 10:37 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-16 10:26 [PATCH mm-unstable v1 00/20] mm/gup: remove FOLL_FORCE usage from drivers (reliable R/O long-term pinning) David Hildenbrand
2022-11-16 10:26 ` [PATCH mm-unstable v1 01/20] selftests/vm: anon_cow: prepare for non-anonymous COW tests David Hildenbrand
2022-11-18 16:20 ` Vlastimil Babka
2022-11-16 10:26 ` [PATCH mm-unstable v1 02/20] selftests/vm: cow: basic COW tests for non-anonymous pages David Hildenbrand
2022-11-16 10:26 ` [PATCH mm-unstable v1 03/20] selftests/vm: cow: R/O long-term pinning reliability tests for non-anon pages David Hildenbrand
2022-11-16 10:26 ` [PATCH mm-unstable v1 04/20] mm: add early FAULT_FLAG_UNSHARE consistency checks David Hildenbrand
2022-11-18 16:45 ` Vlastimil Babka
2022-11-16 10:26 ` [PATCH mm-unstable v1 05/20] mm: add early FAULT_FLAG_WRITE " David Hildenbrand
2022-11-18 17:03 ` Vlastimil Babka
2022-11-16 10:26 ` [PATCH mm-unstable v1 06/20] mm: rework handling in do_wp_page() based on private vs. shared mappings David Hildenbrand
2022-11-22 14:20 ` Vlastimil Babka
2022-11-16 10:26 ` [PATCH mm-unstable v1 07/20] mm: don't call vm_ops->huge_fault() in wp_huge_pmd()/wp_huge_pud() for private mappings David Hildenbrand
2022-11-22 14:50 ` Vlastimil Babka
2022-11-16 10:26 ` [PATCH mm-unstable v1 08/20] mm: extend FAULT_FLAG_UNSHARE support to anything in a COW mapping David Hildenbrand
2022-11-22 15:35 ` Vlastimil Babka
2022-11-16 10:26 ` David Hildenbrand [this message]
2022-11-16 10:42 ` [PATCH mm-unstable v1 09/20] mm/gup: reliable R/O long-term pinning in COW mappings Daniel Vetter
2022-11-22 16:29 ` Vlastimil Babka
2022-11-24 1:29 ` John Hubbard
2022-11-16 10:26 ` [PATCH mm-unstable v1 10/20] RDMA/umem: remove FOLL_FORCE usage David Hildenbrand
2022-11-17 0:45 ` Jason Gunthorpe
2022-11-16 10:26 ` [PATCH mm-unstable v1 11/20] RDMA/usnic: " David Hildenbrand
2022-11-17 0:45 ` Jason Gunthorpe
2022-11-16 10:26 ` [PATCH mm-unstable v1 12/20] RDMA/siw: " David Hildenbrand
2022-11-17 0:46 ` Jason Gunthorpe
2022-11-16 10:26 ` [PATCH mm-unstable v1 13/20] media: videobuf-dma-sg: " David Hildenbrand
2022-11-16 10:48 ` Daniel Vetter
2022-11-23 13:17 ` Hans Verkuil
2022-11-16 10:26 ` [PATCH mm-unstable v1 14/20] drm/etnaviv: " David Hildenbrand
2022-11-16 10:49 ` Daniel Vetter
2022-11-16 10:26 ` [PATCH mm-unstable v1 15/20] media: pci/ivtv: " David Hildenbrand
2022-11-23 13:18 ` Hans Verkuil
2022-11-16 10:26 ` [PATCH mm-unstable v1 16/20] mm/frame-vector: " David Hildenbrand
2022-11-16 10:50 ` Daniel Vetter
2022-11-23 13:26 ` Hans Verkuil
2022-11-23 14:28 ` Hans Verkuil
2022-11-27 10:35 ` David Hildenbrand
2022-11-28 8:17 ` Hans Verkuil
2022-11-28 8:18 ` David Hildenbrand
2022-11-28 8:26 ` Hans Verkuil
2022-11-28 8:57 ` Tomasz Figa
2022-11-28 22:59 ` Andrew Morton
2022-11-29 8:48 ` David Hildenbrand
2022-11-29 9:08 ` Hans Verkuil
2022-11-29 9:15 ` David Hildenbrand
2022-11-16 10:26 ` [PATCH mm-unstable v1 17/20] drm/exynos: " David Hildenbrand
2022-11-16 10:50 ` Daniel Vetter
2022-11-16 10:26 ` [PATCH mm-unstable v1 18/20] RDMA/hw/qib/qib_user_pages: " David Hildenbrand
2022-11-16 10:26 ` [PATCH mm-unstable v1 19/20] habanalabs: " David Hildenbrand
2022-11-16 10:26 ` [PATCH mm-unstable v1 20/20] mm: rename FOLL_FORCE to FOLL_PTRACE David Hildenbrand
2022-11-16 18:16 ` Linus Torvalds
2022-11-16 18:53 ` David Hildenbrand
2022-11-17 22:58 ` Kees Cook
2022-11-17 23:20 ` Linus Torvalds
2022-11-18 0:31 ` Kees Cook
2022-11-18 11:09 ` Peter Zijlstra
2022-11-18 22:29 ` Kees Cook
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