[v2,1/2] btrfs: avoid possible signal interruption for btrfs_drop_snapshot() on relocation tree
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Message ID 20200709083333.137927-1-wqu@suse.com
State New
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Series
  • [v2,1/2] btrfs: avoid possible signal interruption for btrfs_drop_snapshot() on relocation tree
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Commit Message

Qu Wenruo July 9, 2020, 8:33 a.m. UTC
[BUG]
There is a bug report about bad signal timing could lead to read-only
fs during balance:

  BTRFS info (device xvdb): balance: start -d -m -s
  BTRFS info (device xvdb): relocating block group 73001861120 flags metadata
  BTRFS info (device xvdb): found 12236 extents, stage: move data extents
  BTRFS info (device xvdb): relocating block group 71928119296 flags data
  BTRFS info (device xvdb): found 3 extents, stage: move data extents
  BTRFS info (device xvdb): found 3 extents, stage: update data pointers
  BTRFS info (device xvdb): relocating block group 60922265600 flags metadata
  BTRFS: error (device xvdb) in btrfs_drop_snapshot:5505: errno=-4 unknown
  BTRFS info (device xvdb): forced readonly
  BTRFS info (device xvdb): balance: ended with status: -4

[CAUSE]
The direct cause is the -EINTR from the following call chain when a
fatal signal is pending:

 relocate_block_group()
 |- clean_dirty_subvols()
    |- btrfs_drop_snapshot()
       |- btrfs_start_transaction()
          |- btrfs_delayed_refs_rsv_refill()
             |- btrfs_reserve_metadata_bytes()
                |- __reserve_metadata_bytes()
                   |- wait_reserve_ticket()
                      |- prepare_to_wait_event();
                      |- ticket->error = -EINTR;

Normally this behavior is fine for most btrfs_start_transaction()
callers, as they need to catch the fatal signal and exit asap.

However for balance, especially for the clean_dirty_subvols() case, we're
already doing cleanup works, such -EINTR from btrfs_drop_snapshot()
could cause a lot of unexpected problems.

From the mentioned forced read-only, to later balance error due to half
dropped reloc trees.

[FIX]
Fix this problem by using btrfs_join_transaction() if
btrfs_drop_snapshot() is called from relocation context.

As btrfs_join_transaction() won't wait full tickets, it won't get
interrupted from signal.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/extent-tree.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Comments

David Sterba July 9, 2020, 11:05 a.m. UTC | #1
On Thu, Jul 09, 2020 at 04:33:32PM +0800, Qu Wenruo wrote:

> From the mentioned forced read-only, to later balance error due to half
> dropped reloc trees.
> 
> [FIX]
> Fix this problem by using btrfs_join_transaction() if
> btrfs_drop_snapshot() is called from relocation context.
> 
> As btrfs_join_transaction() won't wait full tickets, it won't get
> interrupted from signal.

Could you please rephrase the text above?
Qu Wenruo July 9, 2020, 12:11 p.m. UTC | #2
On 2020/7/9 下午7:05, David Sterba wrote:
> On Thu, Jul 09, 2020 at 04:33:32PM +0800, Qu Wenruo wrote:
> 
>> From the mentioned forced read-only, to later balance error due to half
>> dropped reloc trees.
>>
>> [FIX]
>> Fix this problem by using btrfs_join_transaction() if
>> btrfs_drop_snapshot() is called from relocation context.
>>
>> As btrfs_join_transaction() won't wait full tickets, it won't get
>> interrupted from signal.
> 
> Could you please rephrase the text above?
> 
How about this:

As btrfs_join_transaction() won't reserve new metadata space, it won't
get interrupted by signal at all.

Thanks,
Qu

Patch
diff mbox series

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index c0bc35f932bf..d8ef48a807d1 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -5298,7 +5298,10 @@  int btrfs_drop_snapshot(struct btrfs_root *root, int update_ref, int for_reloc)
 		goto out;
 	}
 
-	trans = btrfs_start_transaction(tree_root, 0);
+	if (for_reloc)
+		trans = btrfs_join_transaction(tree_root);
+	else
+		trans = btrfs_start_transaction(tree_root, 0);
 	if (IS_ERR(trans)) {
 		err = PTR_ERR(trans);
 		goto out_free;