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author | David Hildenbrand <david@redhat.com> | 2022-11-16 13:26:48 +0300 |
---|---|---|
committer | Andrew Morton <akpm@linux-foundation.org> | 2022-12-01 02:58:58 +0300 |
commit | 84209e87c6963f928194a890399e24e8ad299db1 (patch) | |
tree | 987c556289f568ffc17bfa211285bc5a3481ac0c /mm/hugetlb.c | |
parent | 8d6a0ac09a16c026e1e2a03a61e12e95c48a25a6 (diff) | |
download | linux-84209e87c6963f928194a890399e24e8ad299db1.tar.xz |
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 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@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>
Diffstat (limited to 'mm/hugetlb.c')
-rw-r--r-- | mm/hugetlb.c | 7 |
1 files changed, 4 insertions, 3 deletions
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 3d381b26d553..9d97c9a2a15d 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -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; |