Re: [PATCH v2 09/16] ext4: fix zombie groups in average fragment size lists

[Baokun Li <libaokun1@huawei.com>]

On 2025/6/28 3:14, Jan Kara wrote:
> On Mon 23-06-25 15:32:57, Baokun Li wrote:
>> Groups with no free blocks shouldn't be in any average fragment size list.
>> However, when all blocks in a group are allocated(i.e., bb_fragments or
>> bb_free is 0), we currently skip updating the average fragment size, which
>> means the group isn't removed from its previous s_mb_avg_fragment_size[old]
>> list.
>>
>> This created "zombie" groups that were always skipped during traversal as
>> they couldn't satisfy any block allocation requests, negatively impacting
>> traversal efficiency.
>>
>> Therefore, when a group becomes completely free, bb_avg_fragment_size_order
> 					     ^^^ full

Oh, thank you for pointing out that typo!
I'll correct it in the next version.


Thanks,
Baokun

>> is now set to -1. If the old order was not -1, a removal operation is
>> performed; if the new order is not -1, an insertion is performed.
>>
>> Fixes: 196e402adf2e ("ext4: improve cr 0 / cr 1 group scanning")
>> CC: stable@vger.kernel.org
>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> Good catch! The patch looks good. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>
>
> 								Honza
>
>> ---
>>   fs/ext4/mballoc.c | 36 ++++++++++++++++++------------------
>>   1 file changed, 18 insertions(+), 18 deletions(-)
>>
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index 94950b07a577..e6d6c2da3c6e 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -841,30 +841,30 @@ static void
>>   mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
>>   {
>>   	struct ext4_sb_info *sbi = EXT4_SB(sb);
>> -	int new_order;
>> +	int new, old;
>>   
>> -	if (!test_opt2(sb, MB_OPTIMIZE_SCAN) || grp->bb_fragments == 0)
>> +	if (!test_opt2(sb, MB_OPTIMIZE_SCAN))
>>   		return;
>>   
>> -	new_order = mb_avg_fragment_size_order(sb,
>> -					grp->bb_free / grp->bb_fragments);
>> -	if (new_order == grp->bb_avg_fragment_size_order)
>> +	old = grp->bb_avg_fragment_size_order;
>> +	new = grp->bb_fragments == 0 ? -1 :
>> +	      mb_avg_fragment_size_order(sb, grp->bb_free / grp->bb_fragments);
>> +	if (new == old)
>>   		return;
>>   
>> -	if (grp->bb_avg_fragment_size_order != -1) {
>> -		write_lock(&sbi->s_mb_avg_fragment_size_locks[
>> -					grp->bb_avg_fragment_size_order]);
>> +	if (old >= 0) {
>> +		write_lock(&sbi->s_mb_avg_fragment_size_locks[old]);
>>   		list_del(&grp->bb_avg_fragment_size_node);
>> -		write_unlock(&sbi->s_mb_avg_fragment_size_locks[
>> -					grp->bb_avg_fragment_size_order]);
>> -	}
>> -	grp->bb_avg_fragment_size_order = new_order;
>> -	write_lock(&sbi->s_mb_avg_fragment_size_locks[
>> -					grp->bb_avg_fragment_size_order]);
>> -	list_add_tail(&grp->bb_avg_fragment_size_node,
>> -		&sbi->s_mb_avg_fragment_size[grp->bb_avg_fragment_size_order]);
>> -	write_unlock(&sbi->s_mb_avg_fragment_size_locks[
>> -					grp->bb_avg_fragment_size_order]);
>> +		write_unlock(&sbi->s_mb_avg_fragment_size_locks[old]);
>> +	}
>> +
>> +	grp->bb_avg_fragment_size_order = new;
>> +	if (new >= 0) {
>> +		write_lock(&sbi->s_mb_avg_fragment_size_locks[new]);
>> +		list_add_tail(&grp->bb_avg_fragment_size_node,
>> +				&sbi->s_mb_avg_fragment_size[new]);
>> +		write_unlock(&sbi->s_mb_avg_fragment_size_locks[new]);
>> +	}
>>   }
>>   
>>   /*
>> -- 
>> 2.46.1
>>