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SubjectRE: [PATCH] mm, compaction: fix fast_isolate_around() to stay within boundaries
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Sorry, I may not have sent the email correctly. 
I will resend it.

On Thu, 27 Oct 2022 20:26:04 +0000 Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 26 Oct 2022 20:24:38 +0900 NARIBAYASHI Akira <a.naribayashi@fujitsu.com> wrote:
>
> > Depending on the memory configuration, isolate_freepages_block() may
> > scan pages out of the target range and causes panic.
> >
> > The problem is that pfn as argument of fast_isolate_around() could
> > be out of the target range. Therefore we should consider the case
> > where pfn < start_pfn, and also the case where end_pfn < pfn.
> >
> > This problem should have been addressd by the commit 6e2b7044c199
> > ("mm, compaction: make fast_isolate_freepages() stay within zone")
> > but there was an oversight.
> >
> > Case1: pfn < start_pfn
> >
> > <at memory compaction for node Y>
> > | node X's zone | node Y's zone
> > +-----------------+------------------------------...
> > pageblock ^ ^ ^
> > +-----------+-----------+-----------+-----------+...
> > ^ ^ ^
> > ^ ^ end_pfn
> > ^ start_pfn = cc->zone->zone_start_pfn
> > pfn
> > <---------> scanned range by "Scan After"
> >
> > Case2: end_pfn < pfn
> >
> > <at memory compaction for node X>
> > | node X's zone | node Y's zone
> > +-----------------+------------------------------...
> > pageblock ^ ^ ^
> > +-----------+-----------+-----------+-----------+...
> > ^ ^ ^
> > ^ ^ pfn
> > ^ end_pfn
> > start_pfn
> > <---------> scanned range by "Scan Before"
> >
> > It seems that there is no good reason to skip nr_isolated pages
> > just after given pfn. So let perform simple scan from start to end
> > instead of dividing the scan into "Before" and "After".
>
> Under what circumstances will this panic occur? I assume those
> circumstnces are pretty rare, give that 6e2b7044c1992 was nearly two
> years ago.
>
> Did you consider the desirability of backporting this fix into earlier
> kernels?


Panic can occur on systems with multiple zones in a single pageblock.

The reason it is rare is that it only happens in special configurations.
Depending on how many similar systems there are, it may be a good idea to fix this problem for older kernels as well.

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