Re: [PATCH 2/3] btrfs: zoned: fix write time activation failure for metadata block group

[Damien Le Moal <dlemoal@kernel.org>]

On 7/8/25 2:04 PM, Naohiro Aota wrote:
> On Mon Jul 7, 2025 at 12:33 PM JST, Damien Le Moal wrote:
>> On 7/7/25 11:44 AM, Naohiro Aota wrote:
>>> Since commit 13bb483d32ab ("btrfs: zoned: activate metadata block group on
>>> write time"), we activate a metadata block group on the write time. If the
>>
>> on the write time -> at write time
>>
>>> zone capacity is small enough, we can allocate the entire region before the
>>> first write. Then, we hit the btrfs_zoned_bg_is_full() in
>>> btrfs_zone_activate() and the activation fails.
>>>
>>> For a data block group, we already check the fullness condition in the
>>> caller side. And, the fullness check is not necessary for metadata's
>>> write-time activation. Replace it with a proper WARN.
>>
>> I am very confused by this explanation. If the BG is fully allocated before we
>> issue the first write, we still need to activate that BG, no ? So why the
>> WARN() ? That seems wrong to me. But I may not be understanding your
>> explanation, which means you need to clarify it :)
> 
> We activate a data block group before the allocation to simplify the
> write stage. So, activating a full block group should arise WARN.
> 
> OTOH, metadata block group is activated on the write stage. So, it is OK
> to have a full block group to be activated. Instead, it is WARN when we
> try to activate a partially written (meta_write_pointer != bg->start)
> block group.

OK. Please update the commit message with this to clarify.
Also, a comment in the coe would be nice to remind this to whoever read that
code :)

-- 
Damien Le Moal
Western Digital Research