From 213c900d693b87e709e32b046858843fad0332ea Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Sat, 28 May 2011 13:14:09 -0700 Subject: [PATCH] tmpfs: fix race between truncate and writepage commit 826267cf1e6c6899eda1325a19f1b1d15c558b20 upstream. While running fsx on tmpfs with a memhog then swapoff, swapoff was hanging (interruptibly), repeatedly failing to locate the owner of a 0xff entry in the swap_map. Although shmem_writepage() does abandon when it sees incoming page index is beyond eof, there was still a window in which shmem_truncate_range() could come in between writepage's dropping lock and updating swap_map, find the half-completed swap_map entry, and in trying to free it, leave it in a state that swap_shmem_alloc() could not correct. Arguably a bug in __swap_duplicate()'s and swap_entry_free()'s handling of the different cases, but easiest to fix by moving swap_shmem_alloc() under cover of the lock. More interesting than the bug: it's been there since 2.6.33, why could I not see it with earlier kernels? The mmotm of two weeks ago seems to have some magic for generating races, this is just one of three I found. With yesterday's git I first saw this in mainline, bisected in search of that magic, but the easy reproducibility evaporated. Oh well, fix the bug. Signed-off-by: Hugh Dickins Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/shmem.c b/mm/shmem.c index 21bdaf8fd21c..91dd9c3efe87 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1102,8 +1102,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) remove_from_page_cache(page); shmem_swp_set(info, entry, swap.val); shmem_swp_unmap(entry); - spin_unlock(&info->lock); swap_shmem_alloc(swap); + spin_unlock(&info->lock); BUG_ON(page_mapped(page)); page_cache_release(page); /* pagecache ref */ swap_writepage(page, wbc); -- 2.39.5