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

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Dec 09, 2022 at 09:19:37AM +0000, Akira Naribayashi (Fujitsu) wrote:
> On Wed, 23 Nov 2022 10:26:05 +0000, Mei Gorman wrote:
> > On Wed, Nov 09, 2022 at 05:41:12AM +0000, Akira Naribayashi (Fujitsu) wrote:
> > > On Mon, 7 Nov 2022 15:43:56 +0000, Mei Gorman wrote:
> > > > On Mon, Nov 07, 2022 at 12:32:34PM +0000, Akira Naribayashi (Fujitsu) wrote:
> > > > > > 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.
> > > > > 
> > > > 
> > > > Please provide an example of the panic and the zoneinfo.
> > > 
> > > This issue is occurring in our customer's environment and cannot 
> > > be shared publicly as it contains customer information.
> > > Also, the panic is occurring with the kernel in RHEL and may not 
> > > panic with Upstream's community kernel.
> > > In other words, it is possible to panic on older kernels.
> > > I think this fix should be backported to stable kernel series.
> > > 
> > > > > The reason it is rare is that it only happens in special configurations.
> > > > 
> > > > How is this special configuration created?
> > > 
> > > This is the case when the node boundary is not aligned to pageblock boundary.
> > 
> > In that case, does this work to avoid rescanning an area that was already
> > isolated?
> 
> In the case of your patch, I think I need to clamp the isolated_end as well.
> Because sometimes isolated_end < start_pfn(value before entering Scan after) < end_pfn.
> 
> After re-reading the source, I think the problem is that min_pfn and low_pfn
> can be out of range in fast_isolate_freepages.
> How about the following patch?
> 

Ok, makes sense and it is a condition that could happen because of pageblock
alignment.

-- 
Mel Gorman
SUSE Labs