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

[Naohiro Aota <Naohiro.Aota@wdc.com>]

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.

>
>> 
>> Fixes: 13bb483d32ab ("btrfs: zoned: activate metadata block group on write time")
>> CC: stable@vger.kernel.org # 6.6+
>> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
>> ---
>>  fs/btrfs/zoned.c | 11 +++++++----
>>  1 file changed, 7 insertions(+), 4 deletions(-)
>> 
>> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
>> index 6fb4d95412d6..9c354e84ab07 100644
>> --- a/fs/btrfs/zoned.c
>> +++ b/fs/btrfs/zoned.c
>> @@ -2169,10 +2169,13 @@ bool btrfs_zone_activate(struct btrfs_block_group *block_group)
>>  		goto out_unlock;
>>  	}
>>  
>> -	/* No space left */
>> -	if (btrfs_zoned_bg_is_full(block_group)) {
>> -		ret = false;
>> -		goto out_unlock;
>> +	if (block_group->flags & BTRFS_BLOCK_GROUP_DATA) {
>> +		if (WARN_ON_ONCE(btrfs_zoned_bg_is_full(block_group))) {
>> +			ret = false;
>> +			goto out_unlock;
>> +		}
>> +	} else {
>> +		WARN_ON_ONCE(block_group->meta_write_pointer != block_group->start);
>>  	}
>>  
>>  	for (i = 0; i < map->num_stripes; i++) {