diff options
author | Hugh Dickins <hugh@veritas.com> | 2005-06-21 17:15:12 -0700 |
---|---|---|
committer | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-06-21 18:46:21 -0700 |
commit | c475a8ab625d567eacf5e30ec35d6d8704558062 (patch) | |
tree | 0971bef7b876f1b3eb160621fc2b61cb5313827b /mm/rmap.c | |
parent | d296e9cd02c92e576ecce5344026a4df4353cdb2 (diff) | |
download | linux-stable-c475a8ab625d567eacf5e30ec35d6d8704558062.tar.gz linux-stable-c475a8ab625d567eacf5e30ec35d6d8704558062.tar.bz2 linux-stable-c475a8ab625d567eacf5e30ec35d6d8704558062.zip |
[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.c | 21 |
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); |