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authorHugh Dickins <hughd@google.com>2010-10-03 04:49:08 +0400
committerLinus Torvalds <torvalds@linux-foundation.org>2010-10-04 22:09:53 +0400
commit4e31635c367a9e21a43cfbfae4c9deda2e19d1f4 (patch)
tree67e27c7560fbf222958e5d9206f39c3b400be98e /mm/mempolicy.c
parent4829b906cc063cb7cd1b7f34fa05de6db75ec8bb (diff)
downloadlinux-4e31635c367a9e21a43cfbfae4c9deda2e19d1f4.tar.xz
ksm: fix bad user data when swapping
Building under memory pressure, with KSM on 2.6.36-rc5, collapsed with an internal compiler error: typically indicating an error in swapping. Perhaps there's a timing issue which makes it now more likely, perhaps it's just a long time since I tried for so long: this bug goes back to KSM swapping in 2.6.33. Notice how reuse_swap_page() allows an exclusive page to be reused, but only does SetPageDirty if it can delete it from swap cache right then - if it's currently under Writeback, it has to be left in cache and we don't SetPageDirty, but the page can be reused. Fine, the dirty bit will get set in the pte; but notice how zap_pte_range() does not bother to transfer pte_dirty to page_dirty when unmapping a PageAnon. If KSM chooses to share such a page, it will look like a clean copy of swapcache, and not be written out to swap when its memory is needed; then stale data read back from swap when it's needed again. We could fix this in reuse_swap_page() (or even refuse to reuse a page under writeback), but it's more honest to fix my oversight in KSM's write_protect_page(). Several days of testing on three machines confirms that this fixes the issue they showed. Signed-off-by: Hugh Dickins <hughd@google.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: stable@kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/mempolicy.c')
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