Re: [PATCH 3/4] btrfs: fix force usage in inc_block_group_ro

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/11/26 上午10:43, Qu Wenruo wrote:
> 
> 
> On 2019/11/25 下午10:40, Josef Bacik wrote:
>> For some reason we've translated the do_chunk_alloc that goes into
>> btrfs_inc_block_group_ro to force in inc_block_group_ro, but these are
>> two different things.
>>
>> force for inc_block_group_ro is used when we are forcing the block group
>> read only no matter what, for example when the underlying chunk is
>> marked read only.  We need to not do the space check here as this block
>> group needs to be read only.
>>
>> btrfs_inc_block_group_ro() has a do_chunk_alloc flag that indicates that
>> we need to pre-allocate a chunk before marking the block group read
>> only.  This has nothing to do with forcing, and in fact we _always_ want
>> to do the space check in this case, so unconditionally pass false for
>> force in this case.
> 
> I think the patch order makes thing a little hard to grasp here.
> Without the last patch, the idea itself is not correct.
> 
> The reason to force ro is because we want to avoid empty chunk to be
> allocated, especially for scrub case.
> 
> 
> If you put the last patch before this one, it's more clear, as then we
> can accept over-commit, we won't return false ENOSPC and no empty chunk
> created.
> 
> BTW, with the last patch applied, we can remove that @force parameter
> for inc_block_group_ro().

My bad, @force parameter is still needed. Didn't notice that until all
patches applied.

Thanks,
Qu

> 
> Thanks,
> Qu
>>
>> Then fixup inc_block_group_ro to honor force as it's expected and
>> documented to do.
>>
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> ---
>>  fs/btrfs/block-group.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
>> index db539bfc5a52..3ffbc2e0af21 100644
>> --- a/fs/btrfs/block-group.c
>> +++ b/fs/btrfs/block-group.c
>> @@ -1190,8 +1190,10 @@ static int inc_block_group_ro(struct btrfs_block_group *cache, int force)
>>  	spin_lock(&sinfo->lock);
>>  	spin_lock(&cache->lock);
>>  
>> -	if (cache->ro) {
>> +	if (cache->ro || force) {
>>  		cache->ro++;
>> +		if (list_empty(&cache->ro_list))
>> +			list_add_tail(&cache->ro_list, &sinfo->ro_bgs);
>>  		ret = 0;
>>  		goto out;
>>  	}
>> @@ -2063,7 +2065,7 @@ int btrfs_inc_block_group_ro(struct btrfs_block_group *cache,
>>  		}
>>  	}
>>  
>> -	ret = inc_block_group_ro(cache, !do_chunk_alloc);
>> +	ret = inc_block_group_ro(cache, false);
>>  	if (!do_chunk_alloc)
>>  		goto unlock_out;
>>  	if (!ret)
>>
> 


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