The patch titled
tmpfs: fix shmem_swaplist races
has been removed from the -mm tree. Its filename was
tmpfs-fix-shmem_swaplist-races.patch
This patch was dropped because it was merged into mainline or a subsystem tree
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: tmpfs: fix shmem_swaplist races
From: Hugh Dickins <hugh@xxxxxxxxxxx>
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@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/shmem.c | 37 +++++++++++++++++++++++++------------
1 file changed, 25 insertions(+), 12 deletions(-)
diff -puN mm/shmem.c~tmpfs-fix-shmem_swaplist-races mm/shmem.c
--- a/mm/shmem.c~tmpfs-fix-shmem_swaplist-races
+++ a/mm/shmem.c
@@ -833,6 +833,10 @@ static int shmem_unuse_inode(struct shme
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, struc
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 *
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;
}
_
Patches currently in -mm which might be from hugh@xxxxxxxxxxx are
origin.patch
git-unionfs.patch
git-x86.patch
r-o-bind-mounts-track-number-of-mount-writer-fix-buggy-loop.patch
r-o-bind-mounts-track-number-of-mount-writer-fix-buggy-loop-checkpatch-fixes.patch
memcgroup-temporarily-revert-swapoff-mod.patch
memory-controller-memory-accounting-v7.patch
memory-controller-add-per-container-lru-and-reclaim-v7.patch
memcgroup-reinstate-swapoff-mod.patch
memcgroup-fix-zone-isolation-oom.patch
memcgroup-revert-swap_state-mods.patch
memory-controller-use-rcu_read_lock-in-mem_cgroup_cache_charge.patch
memcgroup-tidy-up-mem_cgroup_charge_common.patch
memcgroup-fix-hang-with-shmem-tmpfs.patch
mount-options-fix-tmpfs.patch
mount-options-fix-tmpfs-fix.patch
add-new-string-functions-strict_strto-and-convert-kernel-params-to-use-them-fix-2.patch
prio_tree-debugging-patch.patch
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