[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, 0 insertions, 21 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 9827409eb7c..89770bd25f3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -539,27 +539,6 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma)
 		goto out_unmap;
 	}
 
-	/*
-	 * Don't pull an anonymous page out from under get_user_pages.
-	 * GUP carefully breaks COW and raises page count (while holding
-	 * page_table_lock, as we have here) to make sure that the page
-	 * cannot be freed.  If we unmap that page here, a user write
-	 * access to the virtual address will bring back the page, but
-	 * its raised count will (ironically) be taken to mean it's not
-	 * an exclusive swap page, do_wp_page will replace it by a copy
-	 * page, and the user never get to see the data GUP was holding
-	 * the original page for.
-	 *
-	 * This test is also useful for when swapoff (unuse_process) has
-	 * to drop page lock: its reference to the page stops existing
-	 * ptes from being unmapped, so swapoff can make progress.
-	 */
-	if (PageSwapCache(page) &&
-	    page_count(page) != page_mapcount(page) + 2) {
-		ret = SWAP_FAIL;
-		goto out_unmap;
-	}
-
 	/* Nuke the page table entry. */
 	flush_cache_page(vma, address, page_to_pfn(page));
 	pteval = ptep_clear_flush(vma, address, pte);