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mm/gup: reliable R/O long-term pinning in COW mappings 

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 17839856fd ("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@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand 2022-11-16 11:26:48 +01:00 committed by Andrew Morton
parent 8d6a0ac09a
commit 84209e87c6
4 changed files with 34 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

27
include/linux/mm.h
View File

@ -3149,8 +3149,12 @@ 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
@ -3163,8 +3167,25 @@ 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))

10
mm/gup.c
View File

@ -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_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;
}
@ -2566,7 +2566,7 @@ 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;
}
@ -2632,7 +2632,7 @@ 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;
}
@ -2672,7 +2672,7 @@ 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;
}

2
mm/huge_memory.c
View File

@ -1480,7 +1480,7 @@ 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) &&

7
mm/hugetlb.c
View File

@ -6197,7 +6197,8 @@ 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);
@ -6209,7 +6210,7 @@ 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;
}
@ -6338,7 +6339,7 @@ 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;