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); _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
