diff options
authorHugh Dickins <hughd@google.com>2011-10-31 17:09:43 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-10-31 17:30:51 -0700
commit3d470fc385defa60d9af610f05db8e7f8b4f2f5e (patch)
parent35d8c7ad7208dad5d352c483408e555022750978 (diff)
mm: munlock use mapcount to avoid terrible overhead
A process spent 30 minutes exiting, just munlocking the pages of a large anonymous area that had been alternately mprotected into page-sized vmas: for every single page there's an anon_vma walk through all the other little vmas to find the right one. A general fix to that would be a lot more complicated (use prio_tree on anon_vma?), but there's one very simple thing we can do to speed up the common case: if a page to be munlocked is mapped only once, then it is our vma that it is mapped into, and there's no need whatever to walk through all the others. Okay, there is a very remote race in munlock_vma_pages_range(), if between its follow_page() and lock_page(), another process were to munlock the same page, then page reclaim remove it from our vma, then another process mlock it again. We would find it with page_mapcount 1, yet it's still mlocked in another process. But never mind, that's much less likely than the down_read_trylock() failure which munlocking already tolerates (in try_to_unmap_one()): in due course page reclaim will discover and move the page to unevictable instead. [akpm@linux-foundation.org: add comment] Signed-off-by: Hugh Dickins <hughd@google.com> Cc: Michel Lespinasse <walken@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 9 insertions, 1 deletions
diff --git a/mm/mlock.c b/mm/mlock.c
index 7debb4fdf79b..bd34b3a10852 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -110,7 +110,15 @@ void munlock_vma_page(struct page *page)
if (TestClearPageMlocked(page)) {
dec_zone_page_state(page, NR_MLOCK);
if (!isolate_lru_page(page)) {
- int ret = try_to_munlock(page);
+ int ret = SWAP_AGAIN;
+ /*
+ * Optimization: if the page was mapped just once,
+ * that's our mapping and we don't need to check all the
+ * other vmas.
+ */
+ if (page_mapcount(page) > 1)
+ ret = try_to_munlock(page);
* did try_to_unlock() succeed or punt?