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From:   "Akira Naribayashi (Fujitsu)" <a.naribayashi@fujitsu.com>
To:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>
CC:     "vbabka@suse.cz" <vbabka@suse.cz>,
        "mgorman@techsingularity.net" <mgorman@techsingularity.net>,
        "rientjes@google.com" <rientjes@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "Akira Naribayashi (Fujitsu)" <a.naribayashi@fujitsu.com>
Subject: RE: [PATCH] mm, compaction: fix fast_isolate_around() to stay within
 boundaries
Thread-Topic: [PATCH] mm, compaction: fix fast_isolate_around() to stay within
 boundaries
Thread-Index: AQHY6S3O4xnDu8L77EevMVig1gEQ/64iskiAgAVyM4CAC1JoQA==
Date:   Mon, 7 Nov 2022 12:32:34 +0000
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References: <20221027132557.5f724149bd5753036f41512a@linux-foundation.org>
 <20221031073559.36021-1-a.naribayashi@fujitsu.com>
In-Reply-To: <20221031073559.36021-1-a.naribayashi@fujitsu.com>
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Sorry, I may not have sent the email correctly.=20
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@fujit=
su.com> wrote:
>=20
> > Depending on the memory configuration, isolate_freepages_block() may
> > scan pages out of the target range and causes panic.
> >=20
> > 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.
> >=20
> > This problem should have been addressd by the commit 6e2b7044c199
> > ("mm, compaction: make fast_isolate_freepages() stay within zone")
> > but there was an oversight.
> >=20
> >  Case1: pfn < start_pfn
> >=20
> >   <at memory compaction for node Y>
> >   |  node X's zone  | node Y's zone
> >   +-----------------+------------------------------...
> >    pageblock    ^   ^     ^
> >   +-----------+-----------+-----------+-----------+...
> >                 ^   ^     ^
> >                 ^   ^      end_pfn
> >                 ^    start_pfn =3D cc->zone->zone_start_pfn
> >                  pfn
> >                 <---------> scanned range by "Scan After"
> >=20
> >  Case2: end_pfn < pfn
> >=20
> >   <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"
> >=20
> > 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".
>=20
> Under what circumstances will this panic occur?  I assume those
> circumstnces are pretty rare, give that 6e2b7044c1992 was nearly two
> years ago.
>=20
> 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 f=
ix this problem for older kernels as well.