From c475a8ab625d567eacf5e30ec35d6d8704558062 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 21 Jun 2005 17:15:12 -0700 Subject: [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 Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/swapfile.c | 55 ++++++++++++++++--------------------------------------- 1 file changed, 16 insertions(+), 39 deletions(-) (limited to 'mm/swapfile.c') diff --git a/mm/swapfile.c b/mm/swapfile.c index da48405cd9a3..60cd24a55204 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -276,61 +276,37 @@ void swap_free(swp_entry_t entry) } /* - * Check if we're the only user of a swap page, - * when the page is locked. + * How many references to page are currently swapped out? */ -static int exclusive_swap_page(struct page *page) +static inline int page_swapcount(struct page *page) { - int retval = 0; - struct swap_info_struct * p; + int count = 0; + struct swap_info_struct *p; swp_entry_t entry; entry.val = page->private; p = swap_info_get(entry); if (p) { - /* Is the only swap cache user the cache itself? */ - if (p->swap_map[swp_offset(entry)] == 1) { - /* Recheck the page count with the swapcache lock held.. */ - write_lock_irq(&swapper_space.tree_lock); - if (page_count(page) == 2) - retval = 1; - write_unlock_irq(&swapper_space.tree_lock); - } + /* Subtract the 1 for the swap cache itself */ + count = p->swap_map[swp_offset(entry)] - 1; swap_info_put(p); } - return retval; + return count; } /* * We can use this swap cache entry directly * if there are no other references to it. - * - * Here "exclusive_swap_page()" does the real - * work, but we opportunistically check whether - * we need to get all the locks first.. */ int can_share_swap_page(struct page *page) { - int retval = 0; + int count; - if (!PageLocked(page)) - BUG(); - switch (page_count(page)) { - case 3: - if (!PagePrivate(page)) - break; - /* Fallthrough */ - case 2: - if (!PageSwapCache(page)) - break; - retval = exclusive_swap_page(page); - break; - case 1: - if (PageReserved(page)) - break; - retval = 1; - } - return retval; + BUG_ON(!PageLocked(page)); + count = page_mapcount(page); + if (count <= 1 && PageSwapCache(page)) + count += page_swapcount(page); + return count == 1; } /* @@ -529,9 +505,10 @@ static int unuse_mm(struct mm_struct *mm, if (!down_read_trylock(&mm->mmap_sem)) { /* - * Our reference to the page stops try_to_unmap_one from - * unmapping its ptes, so swapoff can make progress. + * Activate page so shrink_cache is unlikely to unmap its + * ptes while lock is dropped, so swapoff can make progress. */ + activate_page(page); unlock_page(page); down_read(&mm->mmap_sem); lock_page(page); -- cgit v1.2.3-55-g7522