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[BUG]
There are users using autodefrag mount option reporting obvious increase
in IO:
> If I compare the write average (in total, I don't have it per process)
> when taking idle periods on the same machine:
> Linux 5.16:
> without autodefrag: ~ 10KiB/s
> with autodefrag: between 1 and 2MiB/s.
>
> Linux 5.15:
> with autodefrag:~ 10KiB/s (around the same as without
> autodefrag on 5.16)
[CAUSE]
When autodefrag mount option is enabled, btrfs_defrag_file() will be
called with @max_sectors = BTRFS_DEFRAG_BATCH (1024) to limit how many
sectors we can defrag in one try.
And then use the number of sectors defragged to determine if we need to
re-defrag.
But commit b18c3ab2343d ("btrfs: defrag: introduce helper to defrag one
cluster") uses wrong unit to increase @sectors_defragged, which should
be in unit of sector, not byte.
This means, if we have defragged any sector, then @sectors_defragged
will be >= sectorsize (normally 4096), which is larger than
BTRFS_DEFRAG_BATCH.
This makes the @max_sectors check in defrag_one_cluster() to underflow,
rendering the whole @max_sectors check useless.
Thus causing way more IO for autodefrag mount options, as now there is
no limit on how many sectors can really be defragged.
[FIX]
Fix the problems by:
- Use sector as unit when increasing @sectors_defragged
- Include @sectors_defragged > @max_sectors case to break the loop
- Add extra comment on the return value of btrfs_defrag_file()
Reported-by: Anthony Ruhier <aruhier@mailbox.org>
Fixes: b18c3ab2343d ("btrfs: defrag: introduce helper to defrag one cluster")
Link: https://lore.kernel.org/linux-btrfs/0a269612-e43f-da22-c5bc-b34b1b56ebe8@mailbox.org/
CC: stable@vger.kernel.org # 5.16
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
484167da77