Re: [PATCH v2 02/16] ext4: remove unnecessary s_mb_last_start

Baokun Li
2025-06-30T03:32:26Z

On 2025/6/28 2:15, Jan Kara wrote:
> On Mon 23-06-25 15:32:50, Baokun Li wrote:
>> ac->ac_g_ex.fe_start is only used in ext4_mb_find_by_goal(), but STREAM
>> ALLOC is activated after ext4_mb_find_by_goal() fails, so there's no need
>> to update ac->ac_g_ex.fe_start, remove the unnecessary s_mb_last_start.
>>
>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> I'd just note that ac->ac_g_ex.fe_start is also used in
> ext4_mb_collect_stats() so this change may impact the statistics gathered
> there. OTOH it is questionable whether we even want to account streaming
> allocation as a goal hit... Anyway, I'm fine with this, I'd just mention it
> in the changelog.
Yes, I missed ext4_mb_collect_stats(). However, instead of explaining
it in the changelog, I think it would be better to move the current
s_bal_goals update to inside or after ext4_mb_find_by_goal().

Then, we could add another variable, such as s_bal_stream_goals, to
represent the hit count for global goals. This kind of statistic would
help us fine-tune the logic for optimizing inode goals and global goals.

What are your thoughts on this?
> Also one nit below but feel free to add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>
Thanks for your review!
>
>> @@ -2849,7 +2848,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>> /* TBD: may be hot point */
>> spin_lock(&sbi->s_md_lock);
>> ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
>> - ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> Maybe reset ac->ac_g_ex.fe_start to 0 instead of leaving it at some random
> value? Just for the sake of defensive programming...
>
> Honza

ac->ac_g_ex.fe_start holds the inode goal's start position, not a random
value. It's unused after ext4_mb_find_by_goal() (if s_bal_stream_goals is
added). Thus, I see no need for further modification. We can always re-add
it if future requirements change.

Thanks,
Baokun