diff options
author | Hugh Dickins <hugh@veritas.com> | 2008-02-05 06:28:55 (GMT) |
---|---|---|
committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2008-02-05 17:44:16 (GMT) |
commit | 1b1b32f2c6f6bb32535d2da62075b51c980880eb (patch) | |
tree | 686aac685a4c04f085dc17cc1a05910149a04933 /mm/shmem.c | |
parent | b409f9fcf04692c0f603d28c73d2e3dfed27bf54 (diff) | |
download | linux-1b1b32f2c6f6bb32535d2da62075b51c980880eb.tar.xz |
tmpfs: fix shmem_swaplist races
Intensive swapoff testing shows shmem_unuse spinning on an entry in
shmem_swaplist pointing to itself: how does that come about? Days pass...
First guess is this: shmem_delete_inode tests list_empty without taking the
global mutex (so the swapping case doesn't slow down the common case); but
there's an instant in shmem_unuse_inode's list_move_tail when the list entry
may appear empty (a rare case, because it's actually moving the head not the
the list member). So there's a danger of leaving the inode on the swaplist
when it's freed, then reinitialized to point to itself when reused. Fix that
by skipping the list_move_tail when it's a no-op, which happens to plug this.
But this same spinning then surfaces on another machine. Ah, I'd never
suspected it, but shmem_writepage's swaplist manipulation is unsafe: though we
still hold page lock, which would hold off inode deletion if the page were in
pagecache, it doesn't hold off once it's in swapcache (free_swap_and_cache
doesn't wait on locked pages). Hmm: we could put the the inode on swaplist
earlier, but then shmem_unuse_inode could never prune unswapped inodes.
Fix this with an igrab before dropping info->lock, as in shmem_unuse_inode;
though I am a little uneasy about the iput which has to follow - it works, and
I see nothing wrong with it, but it is surprising that shmem inode deletion
may now occur below shmem_writepage. Revisit this fix later?
And while we're looking at these races: the way shmem_unuse tests swapped
without holding info->lock looks unsafe, if we've more than one swap area: a
racing shmem_writepage on another page of the same inode could be putting it
in swapcache, just as we're deciding to remove the inode from swaplist -
there's a danger of going on swap without being listed, so a later swapoff
would hang, being unable to locate the entry. Move that test and removal down
into shmem_unuse_inode, once info->lock is held.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/shmem.c')
-rw-r--r-- | mm/shmem.c | 37 |
1 files changed, 25 insertions, 12 deletions
@@ -833,6 +833,10 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s idx = 0; ptr = info->i_direct; spin_lock(&info->lock); + if (!info->swapped) { + list_del_init(&info->swaplist); + goto lost2; + } limit = info->next_index; size = limit; if (size > SHMEM_NR_DIRECT) @@ -894,8 +898,15 @@ found: inode = igrab(&info->vfs_inode); spin_unlock(&info->lock); - /* move head to start search for next from here */ - list_move_tail(&shmem_swaplist, &info->swaplist); + /* + * Move _head_ to start search for next from here. + * But be careful: shmem_delete_inode checks list_empty without taking + * mutex, and there's an instant in list_move_tail when info->swaplist + * would appear empty, if it were the only one on shmem_swaplist. We + * could avoid doing it if inode NULL; or use this minor optimization. + */ + if (shmem_swaplist.next != &info->swaplist) + list_move_tail(&shmem_swaplist, &info->swaplist); mutex_unlock(&shmem_swaplist_mutex); error = 1; @@ -955,10 +966,7 @@ int shmem_unuse(swp_entry_t entry, struct page *page) mutex_lock(&shmem_swaplist_mutex); list_for_each_safe(p, next, &shmem_swaplist) { info = list_entry(p, struct shmem_inode_info, swaplist); - if (info->swapped) - found = shmem_unuse_inode(info, entry, page); - else - list_del_init(&info->swaplist); + found = shmem_unuse_inode(info, entry, page); cond_resched(); if (found) goto out; @@ -1021,18 +1029,23 @@ 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); + if (list_empty(&info->swaplist)) + inode = igrab(inode); + else + inode = NULL; spin_unlock(&info->lock); - if (list_empty(&info->swaplist)) { - mutex_lock(&shmem_swaplist_mutex); - /* move instead of add in case we're racing */ - list_move_tail(&info->swaplist, &shmem_swaplist); - mutex_unlock(&shmem_swaplist_mutex); - } swap_duplicate(swap); BUG_ON(page_mapped(page)); page_cache_release(page); /* pagecache ref */ set_page_dirty(page); unlock_page(page); + if (inode) { + mutex_lock(&shmem_swaplist_mutex); + /* move instead of add in case we're racing */ + list_move_tail(&info->swaplist, &shmem_swaplist); + mutex_unlock(&shmem_swaplist_mutex); + iput(inode); + } return 0; } |