On 17/03/2021 11:54, David Sterba wrote:
> On Wed, Mar 17, 2021 at 05:57:31PM +0900, Johannes Thumshirn wrote:
>> In btrfs_submit_direct() there's a WAN_ON_ONCE() that will trigger if
>> we're submitting a DIO write on a zoned filesystem but are not using
>> REQ_OP_ZONE_APPEND to submit the IO to the block device.
>> This is a left over from a previous version where btrfs_dio_iomap_begin()
>> didn't use btrfs_use_zone_append() to check for sequential write only
>> zones.
> I can't identify the patch where this got changed. I've landed on
> 544d24f9de73 ("btrfs: zoned: enable zone append writing for direct IO")
> but that adds the btrfs_use_zone_append, the append flag and also the
> warning.

It is an oversight from the development phase. In v11 (I think) I've added
08f455593fff ("btrfs: zoned: cache if block group is on a sequential zone")
and forgot to remove the WARN_ON_ONCE() for 544d24f9de73 ("btrfs: zoned: 
enable zone append writing for direct IO").

When developing auto relocation I got hit by the WARN as a block groups
where relocated to conventional zone and the dio code calls
btrfs_use_zone_append() introduced by 08f455593fff to check if it can use
zone append (a.k.a. if it's a sequential zone) or not and sets the appropriate
flags for iomap.

I've never hit it in testing before, as I was relying on emulation to test
the conventional zones code but this one case wasn't hit, because on emulation
fs_info->max_zone_append_size is 0 and the WARN doesn't trigger either.

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