[PATCH] can_share_swap_page: use page_mapcount

Remember that ironic get_user_pages race?  when the raised page_count on a
page swapped out led do_wp_page to decide that it had to copy on write, so
substituted a different page into userspace.  2.6.7 onwards have Andrea's
solution, where try_to_unmap_one backs out if it finds page_count raised.

Which works, but is unsatisfying (rmap.c has no other page_count heuristics),
and was found a few months ago to hang an intensive page migration test.  A
year ago I was hesitant to engage page_mapcount, now it seems the right fix.

So remove the page_count hack from try_to_unmap_one; and use activate_page in
unuse_mm when dropping lock, to replace its secondary effect of helping
swapoff to make progress in that case.

Simplify can_share_swap_page (now called only on anonymous pages) to check
page_mapcount + page_swapcount == 1: still needs the page lock to stabilize
their (pessimistic) sum, but does not need swapper_space.tree_lock for that.

In do_swap_page, move swap_free and unlock_page below page_add_anon_rmap, to
keep sum on the high side, and correct when can_share_swap_page called.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 1c0a3db78a05..da91b7bf9986 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1686,10 +1686,6 @@ static int do_swap_page(struct mm_struct * mm,
         }
 
         /* The page isn't present yet, go ahead with the fault. */
-                
-        swap_free(entry);
-        if (vm_swap_full())
-                remove_exclusive_swap_page(page);
 
         inc_mm_counter(mm, rss);
         pte = mk_pte(page, vma->vm_page_prot);
@@ -1697,12 +1693,16 @@ static int do_swap_page(struct mm_struct * mm,
                 pte = maybe_mkwrite(pte_mkdirty(pte), vma);
                 write_access = 0;
         }
-        unlock_page(page);
 
         flush_icache_page(vma, page);
         set_pte_at(mm, address, page_table, pte);
         page_add_anon_rmap(page, vma, address);
 
+        swap_free(entry);
+        if (vm_swap_full())
+                remove_exclusive_swap_page(page);
+        unlock_page(page);
+
         if (write_access) {
                 if (do_wp_page(mm, vma, address,
                                 page_table, pmd, pte) == VM_FAULT_OOM)