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

[Mel Gorman]

On Thu, Oct 27, 2022 at 01:25:57PM -0700, Andrew Morton wrote:
> On Wed, 26 Oct 2022 20:24:38 +0900 NARIBAYASHI Akira <a.naribayashi@xxxxxxxxxxx> 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'd also like to see a warning or oops report combined with the
/proc/zoneinfo file of the machine affected. This is to confirm it's an
actual bug and not a suspicion based on code inspection and a simplification
of the code. The answer determines whether this is a -stable candidate
or not.

Both Case 1 and 2 require that the initial pfn started outside the zone
which is unexpected. The clamping on zone boundary in fast_isolate_aropund()
is happening due to pageblock alignment as there is no guarantee that zones
are aligned on a hugepage boundary. pfn itself should have been fine as
it is the PFN of a page that was recently isolated.

The Scan After logic should also be ok. In the context it's called,
nr_isolated is the number of pages that were just isolated so 
pfn + nr_isolated is the end of the free page that was just isolated.

The patch itself should be functionally fine but it rescans a region that has
already been isolated which is a little wasteful but it is straight-forward
and the overhead is probably negligible.

-- 
Mel Gorman
SUSE Labs