The patch titled
Subject: mm/migrate.c: stabilise page count when migrating transparent hugepages
has been added to the -mm tree. Its filename is
mm-migrate-stabilise-page-count-when-migrating-transparent-hugepages.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-migrate-stabilise-page-count-when-migrating-transparent-hugepages.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-migrate-stabilise-page-count-when-migrating-transparent-hugepages.patch
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------------------------------------------------------
From: Will Deacon <will.deacon@xxxxxxx>
Subject: mm/migrate.c: stabilise page count when migrating transparent hugepages
When migrating a transparent hugepage, migrate_misplaced_transhuge_page
guards itself against a concurrent fastgup of the page by checking that
the page count is equal to 2 before and after installing the new pmd.
If the page count changes, then the pmd is reverted back to the original
entry, however there is a small window where the new (possibly writable)
pmd is installed and the underlying page could be written by userspace.
Restoring the old pmd could therefore result in loss of data.
This patch fixes the problem by freezing the page count whilst updating
the page tables, which protects against a concurrent fastgup without the
need to restore the old pmd in the failure case (since the page count can
no longer change under our feet).
Link: http://lkml.kernel.org/r/1497349722-6731-4-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Steve Capper <steve.capper@xxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/migrate.c | 15 ++-------------
1 file changed, 2 insertions(+), 13 deletions(-)
diff -puN mm/migrate.c~mm-migrate-stabilise-page-count-when-migrating-transparent-hugepages mm/migrate.c
--- a/mm/migrate.c~mm-migrate-stabilise-page-count-when-migrating-transparent-hugepages
+++ a/mm/migrate.c
@@ -1916,7 +1916,6 @@ int migrate_misplaced_transhuge_page(str
int page_lru = page_is_file_cache(page);
unsigned long mmun_start = address & HPAGE_PMD_MASK;
unsigned long mmun_end = mmun_start + HPAGE_PMD_SIZE;
- pmd_t orig_entry;
/*
* Rate-limit the amount of data that is being migrated to a node.
@@ -1959,8 +1958,7 @@ int migrate_misplaced_transhuge_page(str
/* Recheck the target PMD */
mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
ptl = pmd_lock(mm, pmd);
- if (unlikely(!pmd_same(*pmd, entry) || page_count(page) != 2)) {
-fail_putback:
+ if (unlikely(!pmd_same(*pmd, entry) || !page_ref_freeze(page, 2))) {
spin_unlock(ptl);
mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
@@ -1982,7 +1980,6 @@ fail_putback:
goto out_unlock;
}
- orig_entry = *pmd;
entry = mk_huge_pmd(new_page, vma->vm_page_prot);
entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
@@ -1999,15 +1996,7 @@ fail_putback:
set_pmd_at(mm, mmun_start, pmd, entry);
update_mmu_cache_pmd(vma, address, &entry);
- if (page_count(page) != 2) {
- set_pmd_at(mm, mmun_start, pmd, orig_entry);
- flush_pmd_tlb_range(vma, mmun_start, mmun_end);
- mmu_notifier_invalidate_range(mm, mmun_start, mmun_end);
- update_mmu_cache_pmd(vma, address, &entry);
- page_remove_rmap(new_page, true);
- goto fail_putback;
- }
-
+ page_ref_unfreeze(page, 2);
mlock_migrate_page(new_page, page);
page_remove_rmap(page, true);
set_page_owner_migrate_reason(new_page, MR_NUMA_MISPLACED);
_
Patches currently in -mm which might be from will.deacon@xxxxxxx are
mm-gup-remove-broken-vm_bug_on_page-compound-check-for-hugepages.patch
mm-page_ref-ensure-page_ref_unfreeze-is-ordered-against-prior-accesses.patch
mm-migrate-stabilise-page-count-when-migrating-transparent-hugepages.patch
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