On Fri, Nov 30, 2018 at 5:12 PM Filipe Manana fdmanana@gmail.com wrote:
On Fri, Nov 30, 2018 at 4:53 PM Josef Bacik josef@toxicpanda.com wrote:
From: Josef Bacik jbacik@fb.com
With my delayed refs patches in place we started seeing a large amount of aborts in __btrfs_free_extent
BTRFS error (device sdb1): unable to find ref byte nr 91947008 parent 0 root 35964 owner 1 offset 0 Call Trace: ? btrfs_merge_delayed_refs+0xaf/0x340 __btrfs_run_delayed_refs+0x6ea/0xfc0 ? btrfs_set_path_blocking+0x31/0x60 btrfs_run_delayed_refs+0xeb/0x180 btrfs_commit_transaction+0x179/0x7f0 ? btrfs_check_space_for_delayed_refs+0x30/0x50 ? should_end_transaction.isra.19+0xe/0x40 btrfs_drop_snapshot+0x41c/0x7c0 btrfs_clean_one_deleted_snapshot+0xb5/0xd0 cleaner_kthread+0xf6/0x120 kthread+0xf8/0x130 ? btree_invalidatepage+0x90/0x90 ? kthread_bind+0x10/0x10 ret_from_fork+0x35/0x40
This was because btrfs_drop_snapshot depends on the root not being modified while it's dropping the snapshot. It will unlock the root node (and really every node) as it walks down the tree, only to re-lock it when it needs to do something. This is a problem because if we modify the tree we could cow a block in our path, which free's our reference to that block. Then once we get back to that shared block we'll free our reference to it again, and get ENOENT when trying to lookup our extent reference to that block in __btrfs_free_extent.
This is ultimately happening because we have delayed items left to be processed for our deleted snapshot _after_ all of the inodes are closed for the snapshot. We only run the delayed inode item if we're deleting the inode, and even then we do not run the delayed insertions or delayed removals. These can be run at any point after our final inode does it's last iput, which is what triggers the snapshot deletion. We can end up with the snapshot deletion happening and then have the delayed items run on that file system, resulting in the above problem.
This problem has existed forever, however my patches made it much easier to hit as I wake up the cleaner much more often to deal with delayed iputs, which made us more likely to start the snapshot dropping work before the transaction commits, which is when the delayed items would generally be run. Before, generally speaking, we would run the delayed items, commit the transaction, and wakeup the cleaner thread to start deleting snapshots, which means we were less likely to hit this problem. You could still hit it if you had multiple snapshots to be deleted and ended up with lots of delayed items, but it was definitely harder.
Fix for now by simply running all the delayed items before starting to drop the snapshot. We could make this smarter in the future by making the delayed items per-root, and then simply drop any delayed items for roots that we are going to delete. But for now just a quick and easy solution is the safest.
Cc: stable@vger.kernel.org Signed-off-by: Josef Bacik josef@toxicpanda.com
Reviewed-by: Filipe Manana fdmanana@suse.com
Great catch! I've hit this error from __btrfs_free_extent() a handful of times over the years, but never managed to reproduce it on demand or figure out it was related to snapshot deletion.
fs/btrfs/extent-tree.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index dcb699dd57f3..965702034b22 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -9330,6 +9330,8 @@ int btrfs_drop_snapshot(struct btrfs_root *root, goto out_free; }
btrfs_run_delayed_items(trans);
Btw, we should check the return value of this and return it if it's an error? We can't do nothing with it in the context of the cleaner thread, which is why, I suppose, you chose to ignore the value (besides that the error might have been for some other root). But this can be used in the context of relocation, where we can return the error back to userspace.
Thanks.
if (block_rsv) trans->block_rsv = block_rsv;-- 2.14.3
-- Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”