Re: [RFC 04/11] ext4: Convert mballoc cr (criteria) to enum

Ojaswin Mujoo
2023-03-17T10:27:58Z

On Thu, Mar 09, 2023 at 01:11:22PM +0100, Jan Kara wrote:
> On Fri 27-01-23 18:07:31, Ojaswin Mujoo wrote:
> > Convert criteria to be an enum so it easier to maintain. This change
> > also makes it easier to insert new criterias in the future.
> >
> > Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>
> Just two small comments below:
Hi Jan,

Thanks for the review.
>
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index b8b00457da8d..6037b8e0af86 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -126,6 +126,14 @@ enum SHIFT_DIRECTION {
> > SHIFT_RIGHT,
> > };
> >
> > +/*
> > + * Number of criterias defined. For each criteria, mballoc has slightly
> > + * different way of finding the required blocks nad usually, higher the
> ^^^ and
>
> > + * criteria the slower the allocation. We start at lower criterias and keep
> > + * falling back to higher ones if we are not able to find any blocks.
> > + */
> > +#define EXT4_MB_NUM_CRS 4
> > +
>
> So defining this in a different header than the enum itself is fragile. I
> understand you need it in ext4_sb_info declaration so probably I'd move the
> enum declaration to ext4.h. Alternatively I suppose we could move a lot of
Got it, I'll try to keep them in the same file.

> mballoc stuff out of ext4_sb_info into a separate struct because there's a
> lot of it. But that would be much larger undertaking.
Right, we did notice that as well, but as you said, that's out of scope
of this patchset.
>
> Also when going for symbolic allocator scan names maybe we could actually
> make names sensible instead of CR[0-4]? Perhaps like CR_ORDER2_ALIGNED,
> CR_BEST_LENGHT_FAST, CR_BEST_LENGTH_ALL, CR_ANY_FREE. And probably we could
> deal with ordered comparisons like in:
I like this idea, it should make the code a bit more easier to
understand. However just wondering if I should do it as a part of this
series or a separate patch since we'll be touching code all around and
I don't want to confuse people with the noise :)
>
> if (cr < 2 &&
> (!sbi->s_log_groups_per_flex ||
> ((group & ((1 << sbi->s_log_groups_per_flex) - 1)) != 0)) &
> !(ext4_has_group_desc_csum(sb) &&
> (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))))
> return 0;
>
> to declare CR_FAST_SCAN = 2, or something like that. What do you think?
About this, wont it be better to just use something like

cr < CR_BEST_LENGTH_ALL

instead of defining a new CR_FAST_SCAN = 2.

The only concern is that if we add a new "fast" CR (say between
CR_BEST_LENGTH_FAST and CR_BEST_LENGTH_ALL) then we'll need to make
sure we also update CR_FAST_SCAN to 3 which is easy to miss.

Regards,
Ojaswin
>
> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR