The patch titled Subject: mm: migrate: check page_count of THP before migrating has been added to the -mm tree. Its filename is mm-migrate-check-page_count-of-thp-before-migrating.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Mel Gorman <mgorman@xxxxxxx> Subject: mm: migrate: check page_count of THP before migrating Hugh Dickins pointed out that migrate_misplaced_transhuge_page() does not check page_count before migrating like base page migration and khugepage. He could not see why this was safe and he is right. The potential impact of the bug is avoided due to the limitations of NUMA balancing. The page_mapcount() check ensures that only a single address space is using this page and as THPs are typically private it should not be possible for another address space to fault it in parallel. If the address space has one associated task then it's difficult to have both a GUP pin and be referencing the page at the same time. If there are multiple tasks then a buggy scenario requires that another thread be accessing the page while the direct IO is in flight. This is dodgy behaviour as there is a possibility of corruption with or without THP migration. It would be While we happen to be safe for the most part it is shoddy to depend on such "safety" so this patch checks the page count similar to anonymous pages. Note that this does not mean that the page_mapcount() check can go away. If we were to remove the page_mapcount() check the the THP would have to be unmapped from all referencing PTEs, replaced with migration PTEs and restored properly afterwards. Signed-off-by: Mel Gorman <mgorman@xxxxxxx> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/migrate.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff -puN mm/migrate.c~mm-migrate-check-page_count-of-thp-before-migrating mm/migrate.c --- a/mm/migrate.c~mm-migrate-check-page_count-of-thp-before-migrating +++ a/mm/migrate.c @@ -1679,9 +1679,18 @@ int migrate_misplaced_transhuge_page(str page_xchg_last_nid(new_page, page_last_nid(page)); isolated = numamigrate_isolate_page(pgdat, page); - if (!isolated) { + + /* + * Failing to isolate or a GUP pin prevents migration. The expected + * page count is 2. 1 for anonymous pages without a mapping and 1 + * for the callers pin. If the page was isolated, the page will + * need to be put back on the LRU. + */ + if (!isolated || page_count(page) != 2) { count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR); put_page(new_page); + if (isolated) + putback_lru_page(page); goto out_keep_locked; } _ Patches currently in -mm which might be from mgorman@xxxxxxx are origin.patch linux-next.patch mm-migrate-check-page_count-of-thp-before-migrating.patch mm-memcg-only-evict-file-pages-when-we-have-plenty.patch mm-vmscan-save-work-scanning-almost-empty-lru-lists.patch mm-vmscan-clarify-how-swappiness-highest-priority-memcg-interact.patch mm-vmscan-improve-comment-on-low-page-cache-handling.patch mm-vmscan-clean-up-get_scan_count.patch mm-vmscan-clean-up-get_scan_count-fix.patch mm-vmscan-compaction-works-against-zones-not-lruvecs.patch mm-vmscan-compaction-works-against-zones-not-lruvecs-fix.patch mm-reduce-rmap-overhead-for-ex-ksm-page-copies-created-on-swap-faults.patch mm-page_allocc-__setup_per_zone_wmarks-make-min_pages-unsigned-long.patch mm-vmscanc-__zone_reclaim-replace-max_t-with-max.patch mm-compaction-do-not-accidentally-skip-pageblocks-in-the-migrate-scanner.patch mm-huge_memory-use-new-hashtable-implementation.patch mmksm-use-new-hashtable-implementation.patch mm-memmap_init_zone-performance-improvement.patch -- 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