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

[Joonwon Kang <joonwonkang@google.com>]

> 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>

I have just encountered a kernel panic with this patch. Will submit a new
version after working on it.

Thanks,
Joonwon Kang