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?