The quilt patch titled
Subject: mm/gup: reliable R/O long-term pinning in COW mappings
has been removed from the -mm tree. Its filename was
mm-gup-reliable-r-o-long-term-pinning-in-cow-mappings.patch
This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/gup: reliable R/O long-term pinning in COW mappings
Date: Wed, 16 Nov 2022 11:26:48 +0100
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.
Link: https://lkml.kernel.org/r/20221116102659.70287-10-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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(-)
--- a/include/linux/mm.h~mm-gup-reliable-r-o-long-term-pinning-in-cow-mappings
+++ a/include/linux/mm.h
@@ -3149,8 +3149,12 @@ static inline int vm_fault_to_errno(vm_f
* 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
@@ -3163,8 +3167,25 @@ static inline bool gup_must_unshare(unsi
* 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))
--- a/mm/gup.c~mm-gup-reliable-r-o-long-term-pinning-in-cow-mappings
+++ a/mm/gup.c
@@ -603,7 +603,7 @@ retry:
}
}
- 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;
}
@@ -2380,7 +2380,7 @@ static int gup_pte_range(pmd_t pmd, pmd_
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;
}
@@ -2566,7 +2566,7 @@ static int gup_hugepte(pte_t *ptep, unsi
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;
}
@@ -2632,7 +2632,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_
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;
}
@@ -2672,7 +2672,7 @@ static int gup_huge_pud(pud_t orig, pud_
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;
}
--- a/mm/huge_memory.c~mm-gup-reliable-r-o-long-term-pinning-in-cow-mappings
+++ a/mm/huge_memory.c
@@ -1480,7 +1480,7 @@ struct page *follow_trans_huge_pmd(struc
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) &&
--- a/mm/hugetlb.c~mm-gup-reliable-r-o-long-term-pinning-in-cow-mappings
+++ a/mm/hugetlb.c
@@ -6197,7 +6197,8 @@ static void record_subpages_vmas(struct
}
}
-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);
@@ -6209,7 +6210,7 @@ static inline bool __follow_hugetlb_must
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;
}
@@ -6338,7 +6339,7 @@ long follow_hugetlb_page(struct mm_struc
* 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;
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
selftests-vm-add-ksm-unmerge-tests.patch
mm-pagewalk-dont-trigger-test_walk-in-walk_page_vma.patch
selftests-vm-add-test-to-measure-madv_unmergeable-performance.patch
mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
mm-remove-vm_fault_write.patch
mm-ksm-fix-ksm-cow-breaking-with-userfaultfd-wp-via-fault_flag_unshare.patch
mm-pagewalk-add-walk_page_range_vma.patch
mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch
mm-gup-remove-foll_migration.patch
