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

Andrew Morton
2022-10-27T20:26:02Z

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?