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authorHugh Dickins <hugh@veritas.com>2005-06-21 17:15:12 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-06-21 18:46:21 -0700
commitc475a8ab625d567eacf5e30ec35d6d8704558062 (patch)
tree0971bef7b876f1b3eb160621fc2b61cb5313827b /mm/rmap.c
parentd296e9cd02c92e576ecce5344026a4df4353cdb2 (diff)
[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>
Diffstat (limited to 'mm/rmap.c')
-rw-r--r--mm/rmap.c21
1 files changed, 0 insertions, 21 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index 9827409eb7c7..89770bd25f31 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);