Re: [PATCH v2] percpu: Fix hint invariant breakage

[Joonwon Kang <joonwonkang@google.com>]

> Hi Joonwon,
> 
> On Wed, Apr 08, 2026 at 10:06:42AM +0000, Joonwon Kang wrote:
> > The invariant "scan_hint_start > contig_hint_start if and only if
> > scan_hint == contig_hint" should be kept for hint management. However,
> > it could be broken in some cases:
> > 
> >   - if (new contig == contig_hint == scan_hint) && (contig_hint_start <
> >     scan_hint_start < new contig start) && the new contig is to become a
> >     new contig_hint due to its better alignment, then scan_hint should
> >     be invalidated instead of keeping the old value.
> > 
> >   - if (new contig == contig_hint > scan_hint) && (new contig start <
> >     contig_hint_start) && the new contig is not to become a new
> >     contig_hint, then scan_hint should be not updated to the new contig.
> > 
> > This commit refactors the percpu block update code to make it more
> > visible on what to consider, e.g. when the new contig overlaps with the
> > old contig_hint or scan_hint, fixes the invariant breakage and also
> > optimizes scan_hint further. Some of the optimization cases when no
> > overlap occurs are:
> > 
> >   - if (new contig > contig_hint > scan_hint) && (scan_hint_start < new
> >     contig start < contig_hint_start), then keep scan_hint instead of
> >     invalidating it.
> > 
> >   - if (new contig > contig_hint == scan_hint) && (contig_hint_start <
> >     new contig start < scan_hint_start), then update scan_hint to the
> >     old contig_hint instead of invalidating it.
> > 
> >   - if (new contig == contig_hint > scan_hint) && (new contig start <
> >     contig_hint_start) && the new contig is to become a new contig_hint
> >     due to its better alignment, then update scan_hint to the old
> >     contig_hint instead of invalidating or keeping it.
> > 
> > Signed-off-by: Joonwon Kang <joonwonkang@google.com>
> > ---
> > v1 -> v2: Consider cases where the new contig overlaps with the existing
> >   contig_hint or scan_hint.
> > 
> >  mm/percpu.c | 124 +++++++++++++++++++++++++++++++++++-----------------
> >  1 file changed, 85 insertions(+), 39 deletions(-)
> > 
> 
> Just a few minor style nits; checkpatch.pl reported the following:
> 
> CHECK: Lines should not end with a '('
> #173: FILE: mm/percpu.c:653:
> +       overlap_with_contig_hint = pcpu_region_overlap(
> 
> CHECK: Unnecessary parentheses around 'scan_hint_cand_2 > scan_hint_cand_1'
> #258: FILE: mm/percpu.c:699:
> +       if ((scan_hint_cand_2 > scan_hint_cand_1) ||
> +           (scan_hint_cand_2 == scan_hint_cand_1 &&
> +            scan_hint_cand_2_start > scan_hint_cand_1_start)) {
> 
> Since this patch will need a respin anyway, I think it would be better
> to address these together in the next version.
> 
> Regards,
> Kuan-Wei

Hey, thanks for running style checker for me. As it looked fine when I ran it,
I guess you may have run it with `--strict` option. I am not sure if the strict
option is required. I will keep it as-is since it does not seem very important
unless it hurts readability. Please let me know if you think it is.

Thanks,
Joonwon Kang