From f4e177d12686bf98b5a047b5187121a71ee0dd8c Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 10 Jul 2017 15:48:31 -0700 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@arm.com Signed-off-by: Will Deacon Acked-by: Kirill A. Shutemov Cc: Mel Gorman Cc: Mark Rutland Cc: Steve Capper Cc: Vlastimil Babka Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/migrate.c | 15 ++------------- 1 file changed, 2 insertions(+), 13 deletions(-) (limited to 'mm') diff --git a/mm/migrate.c b/mm/migrate.c index 8935cbe362ce..627671551873 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -1916,7 +1916,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm, 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(struct mm_struct *mm, /* 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); -- cgit v1.2.3