[PATCH] Btrfs: fix a crash when running balance and defrag concurrently

[PATCH] Btrfs: fix a crash when running balance and defrag concurrently

[Liu Bo]

Running balance and defrag concurrently can end up with a crash:

kernel BUG at fs/btrfs/relocation.c:4528!
RIP: 0010:[<ffffffffa01ac33b>]  [<ffffffffa01ac33b>] btrfs_reloc_cow_block+ 0x1eb/0x230 [btrfs]
Call Trace:
  [<ffffffffa01398c1>] ? update_ref_for_cow+0x241/0x380 [btrfs]
  [<ffffffffa0180bad>] ? copy_extent_buffer+0xad/0x110 [btrfs]
  [<ffffffffa0139da1>] __btrfs_cow_block+0x3a1/0x520 [btrfs]
  [<ffffffffa013a0b6>] btrfs_cow_block+0x116/0x1b0 [btrfs]
  [<ffffffffa013ddad>] btrfs_search_slot+0x43d/0x970 [btrfs]
  [<ffffffffa0153c57>] btrfs_lookup_file_extent+0x37/0x40 [btrfs]
  [<ffffffffa0172a5e>] __btrfs_drop_extents+0x11e/0xae0 [btrfs]
  [<ffffffffa013b3fd>] ? generic_bin_search.constprop.39+0x8d/0x1a0 [btrfs]
  [<ffffffff8117d14a>] ? kmem_cache_alloc+0x1da/0x200
  [<ffffffffa0138e7a>] ? btrfs_alloc_path+0x1a/0x20 [btrfs]
  [<ffffffffa0173ef0>] btrfs_drop_extents+0x60/0x90 [btrfs]
  [<ffffffffa016b24d>] relink_extent_backref+0x2ed/0x780 [btrfs]
  [<ffffffffa0162fe0>] ? btrfs_submit_bio_hook+0x1e0/0x1e0 [btrfs]
  [<ffffffffa01b8ed7>] ? iterate_inodes_from_logical+0x87/0xa0 [btrfs]
  [<ffffffffa016b909>] btrfs_finish_ordered_io+0x229/0xac0 [btrfs]
  [<ffffffffa016c3b5>] finish_ordered_fn+0x15/0x20 [btrfs]
  [<ffffffffa018cbe5>] worker_loop+0x125/0x4e0 [btrfs]
  [<ffffffffa018cac0>] ? btrfs_queue_worker+0x300/0x300 [btrfs]
  [<ffffffff81075ea0>] kthread+0xc0/0xd0
  [<ffffffff81075de0>] ? insert_kthread_work+0x40/0x40
  [<ffffffff8164796c>] ret_from_fork+0x7c/0xb0
  [<ffffffff81075de0>] ? insert_kthread_work+0x40/0x40
----------------------------------------------------------------------

It turns out to be that balance operation will bump root's @last_snapshot,
which enables snapshot-aware defrag path, and backref walking stuff will
find data reloc tree as refs' parent, and hit the BUG_ON() during COW.

As data reloc tree's data is just for relocation purpose, and will be deleted right
after relocation is done, it's unnecessary to walk those refs belonged to data reloc
tree, it'd be better to skip them.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
 fs/btrfs/backref.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 0552a59..5eb50b5 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -185,6 +185,9 @@ static int __add_prelim_ref(struct list_head *head, u64 root_id,
 {
 	struct __prelim_ref *ref;
 
+	if (root_id == BTRFS_DATA_RELOC_TREE_OBJECTID)
+		return 0;
+
 	ref = kmem_cache_alloc(btrfs_prelim_ref_cache, gfp_mask);
 	if (!ref)
 		return -ENOMEM;
-- 
1.8.2.1

Re: [PATCH] Btrfs: fix a crash when running balance and defrag concurrently

[Chris Samuel]

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On Wed, 30 Oct 2013 01:25:24 PM Liu Bo wrote:

> Running balance and defrag concurrently can end up with a crash:

It's worth noting that Greg K-H has said he'll take stable patches before they 
hit mainline whilst Linus is off-net, as reported here at the kernel summit:

https://lwn.net/Articles/571997/

# Greg added that, during this time, urgent stable fixes could,
# contrary to normal policy, get into the stable tree prior to
# being merged into the mainline; he won't hold them while
# Linus is off the net.

This patch seems like a very good candidate for that.

cheers!
Chris
-- 
 Chris Samuel  :  http://www.csamuel.org/  :  Melbourne, VIC

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