[PATCH mm-unstable v1 09/20] mm/gup: reliable R/O long-term pinning in COW mappings

David Hildenbrand
2022-11-16T10:38:56Z

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