[PATCH] btrfs: device stat, log when zeroed assist audit

[Anand Jain <anand.jain@oracle.com>]

We had a report indicating that some read errors aren't reported by
the device stats in the userland. It is important to have the errors
reported in the device stat as user land scripts might depend on it to
take the reasonable corrective actions. But to debug these issue we need
to be really sure that request to reset the device stat did not come
from the userland itself. So log an info message when device error reset
happens.

For example:
 BTRFS info (device sdc): device stats zeroed by btrfs (9223)

Reported-by: philip@philip-seeger.de
Link: https://www.spinics.net/lists/linux-btrfs/msg96528.html
Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 BTRFS info (device sdc): device stats zeroed by btrfs (9223)
The last words are name and pid of the process, unfortunately it came out
as 'by btrfs'. At some point if there is a python and lib to reset it
would change, otherwise its going to be 'by btrfs', I am ok with it,
if otherwise please suggest the alternative.

 fs/btrfs/volumes.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index eb55df0d4038..6fd90270e2c7 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7324,6 +7324,8 @@ int btrfs_get_dev_stats(struct btrfs_fs_info *fs_info,
 			else
 				btrfs_dev_stat_set(dev, i, 0);
 		}
+		btrfs_info(fs_info, "device stats zeroed by %s (%d)",
+			   current->comm, task_pid_nr(current));
 	} else {
 		for (i = 0; i < BTRFS_DEV_STAT_VALUES_MAX; i++)
 			if (stats->nr_items > i)
-- 
2.23.0