[PATCH] mm: further fix swapin race condition

[Hugh Dickins <hughd@google.com>]

Commit 4969c1192d15afa3389e7ae3302096ff684ba655 "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>
Cc: stable@kernel.org
---

 mm/memory.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- 2.6.36-rc4/mm/memory.c	2010-09-12 17:34:03.000000000 -0700
+++ linux/mm/memory.c	2010-09-19 18:23:43.000000000 -0700
@@ -2680,10 +2680,12 @@ 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)) {