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authorMel Gorman <mgorman@suse.de>2013-01-11 14:31:40 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2013-01-11 14:54:54 -0800
commit04fa5d6a6547fbfcf613efd00637666fe19b24ab (patch)
tree5ef002ac18663361e53e323e0c9405d0c3f4593c
parent0a1af1d61edae189b0a81bc46386ab37eb3d9d4d (diff)
downloadlinux-04fa5d6a6547fbfcf613efd00637666fe19b24ab.tar.bz2
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@suse.de> Reported-by: Hugh Dickins <hughd@google.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Acked-by: Hugh Dickins <hughd@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/migrate.c14
1 files changed, 13 insertions, 1 deletions
diff --git a/mm/migrate.c b/mm/migrate.c
index 3b676b0c5c3e..c38778610aa8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1679,9 +1679,21 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
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);
+ isolated = 0;
+ goto out;
+ }
goto out_keep_locked;
}