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

Ojaswin Mujoo
2025-07-14T07:02:13Z

On Mon, Jun 30, 2025 at 03:52:58PM +0800, Baokun Li wrote:
> On 2025/6/30 15:31, Jan Kara wrote:
> > On Mon 30-06-25 11:32:16, Baokun Li wrote:
> > > 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?
> > Sure that sounds good to me.
>
> Ok, I will add a patch to implement that logic in the next version.
>
> >
> > > > > @@ -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...
> > > >
> > > 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.
> > Yeah, I was imprecise. It is not a random value. But it is not an offset in
> > the group we are now setting. Therefore I'd still prefer to reset fe_start
> > to 0 (or some invalid value like -1 to catch unexpected use).
> >
> > Honza
>
> When ext4_mb_regular_allocator() fails, it might retry and get called
> again. In this scenario, we can't reliably determine if ac_g_ex has
> already been modified. Therefore, it might be more appropriate to set
> ac_g_ex.fe_start to -1 after ext4_mb_find_by_goal() fails. We can then
> skip ext4_mb_find_by_goal() when ac_g_ex.fe_start < 0.

Hmm idk if giving a sort of one-off special meaning to -1 would be right.

How about resetting the original goal group and goal start in the retry
logic of ext4_mb_new_blocks()? Since we drop preallocations before
retrying, this way we might actually find our goal during the retry
(slim chance though but still).

Regards,
ojaswin
>
>
> Cheers,
> Baokun
>