RE: [PATCH] mm, compaction: fix fast_isolate_around() to stay within boundaries

["Akira Naribayashi (Fujitsu)" <a.naribayashi@fujitsu.com>]

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.