Messages in this thread Patch in this message | | | Date | Fri, 24 Sep 2010 09:24:13 -0700 | From | Greg KH <> | Subject | [25/80] mm: further fix swapin race condition |
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2.6.35-stable review patch. If anyone has any objections, please let us know.
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From: Hugh Dickins <hughd@google.com>
commit 31c4a3d3a0f84a5847665f8aa0552d188389f791 upstream.
Commit 4969c1192d15 ("mm: fix swapin race condition") is now agreed to be incomplete. There's a race, not very much less likely than the original race envisaged, in which it is further necessary to check that the swapcache page's swap has not changed.
Here's the reasoning: cast in terms of reuse_swap_page(), but probably could be reformulated to rely on try_to_free_swap() instead, or on swapoff+swapon.
A, faults into do_swap_page(): does page1 = lookup_swap_cache(swap1) and comes through the lock_page(page1).
B, a racing thread of the same process, faults on the same address: does page1 = lookup_swap_cache(swap1) and now waits in lock_page(page1), but for whatever reason is unlucky not to get the lock any time soon.
A carries on through do_swap_page(), a write fault, but cannot reuse the swap page1 (another reference to swap1). Unlocks the page1 (but B doesn't get it yet), does COW in do_wp_page(), page2 now in that pte.
C, perhaps the parent of A+B, comes in and write faults the same swap page1 into its mm, reuse_swap_page() succeeds this time, swap1 is freed.
kswapd comes in after some time (B still unlucky) and swaps out some pages from A+B and C: it allocates the original swap1 to page2 in A+B, and some other swap2 to the original page1 now in C. But does not immediately free page1 (actually it couldn't: B holds a reference), leaving it in swap cache for now.
B at last gets the lock on page1, hooray! Is PageSwapCache(page1)? Yes. Is pte_same(*page_table, orig_pte)? Yes, because page2 has now been given the swap1 which page1 used to have. So B proceeds to insert page1 into A+B's page_table, though its content now belongs to C, quite different from what A wrote there.
B ought to have checked that page1's swap was still swap1.
Signed-off-by: Hugh Dickins <hughd@google.com> Reviewed-by: Rik van Riel <riel@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- mm/memory.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
--- a/mm/memory.c +++ b/mm/memory.c @@ -2682,10 +2682,12 @@ static int do_swap_page(struct mm_struct delayacct_clear_flag(DELAYACCT_PF_SWAPIN); /* - * Make sure try_to_free_swap didn't release the swapcache - * from under us. The page pin isn't enough to prevent that. + * Make sure try_to_free_swap or reuse_swap_page or swapoff did not + * release the swapcache from under us. The page pin, and pte_same + * test below, are not enough to exclude that. Even if it is still + * swapcache, we need to check that the page's swap has not changed. */ - if (unlikely(!PageSwapCache(page))) + if (unlikely(!PageSwapCache(page) || page_private(page) != entry.val)) goto out_page; if (ksm_might_need_to_copy(page, vma, address)) {
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