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| author | Hugh Dickins <hughd@google.com> | 2010-09-19 19:40:22 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2010-09-20 10:44:37 -0700 |
| commit | 31c4a3d3a0f84a5847665f8aa0552d188389f791 (patch) | |
| tree | 6dbc630213c899c82030e38c9fa1125c060ef2fe /mm/memory.c | |
| parent | 2422084a94fcd5038406261b331672a13c92c050 (diff) | |
| download | lwn-31c4a3d3a0f84a5847665f8aa0552d188389f791.tar.gz lwn-31c4a3d3a0f84a5847665f8aa0552d188389f791.zip | |
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>
Diffstat (limited to 'mm/memory.c')
| -rw-r--r-- | mm/memory.c | 8 |
1 files changed, 5 insertions, 3 deletions
diff --git a/mm/memory.c b/mm/memory.c index 71b161b73bb5..0e18b4d649ec 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)) { |