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 <hughd@xxxxxxxxxx> Cc: stable@xxxxxxxxxx --- mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- linux.orig/mm/shmem.c 2011-05-27 19:05:27.000000000 -0700 +++ linux/mm/shmem.c 2011-05-27 19:45:44.194813695 -0700 @@ -1114,8 +1114,8 @@ static int shmem_writepage(struct page * delete_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)); swap_writepage(page, wbc); return 0; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>