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authorHugh Dickins <hughd@google.com>2010-09-19 19:40:22 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-09-20 10:44:37 -0700
commit31c4a3d3a0f84a5847665f8aa0552d188389f791 (patch)
tree6dbc630213c899c82030e38c9fa1125c060ef2fe
parent2422084a94fcd5038406261b331672a13c92c050 (diff)
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mm: further fix swapin race condition
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> Cc: stable@kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/memory.c8
1 files changed, 5 insertions, 3 deletions
diff --git a/mm/memory.c b/mm/memory.c
index 71b161b73bb..0e18b4d649e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2680,10 +2680,12 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
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)) {