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Message-ID: <2d9312bf-a5ad-0427-c197-fef6ea5cfe0a@suse.cz>
Date: Fri, 1 Sep 2023 10:23:23 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Matthew Wilcox <willy@...radead.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
Huacai Chen <chenhuacai@...nel.org>,
Michael Ellerman <mpe@...erman.id.au>,
Anup Patel <anup@...infault.org>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Andrew Morton <akpm@...ux-foundation.org>,
Paul Moore <paul@...l-moore.com>,
James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>, kvm@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
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Chao Peng <chao.p.peng@...ux.intel.com>,
Fuad Tabba <tabba@...gle.com>,
Jarkko Sakkinen <jarkko@...nel.org>,
Yu Zhang <yu.c.zhang@...ux.intel.com>,
Vishal Annapurve <vannapurve@...gle.com>,
Ackerley Tng <ackerleytng@...gle.com>,
Maciej Szmigiero <mail@...iej.szmigiero.name>,
David Hildenbrand <david@...hat.com>,
Quentin Perret <qperret@...gle.com>,
Michael Roth <michael.roth@....com>,
Wang <wei.w.wang@...el.com>,
Liam Merwick <liam.merwick@...cle.com>,
Isaku Yamahata <isaku.yamahata@...il.com>
Subject: Re: [RFC PATCH v11 10/29] mm: Add AS_UNMOVABLE to mark mapping as
completely unmovable
On 7/25/23 14:51, Matthew Wilcox wrote:
> On Tue, Jul 25, 2023 at 01:24:03PM +0300, Kirill A . Shutemov wrote:
>> On Tue, Jul 18, 2023 at 04:44:53PM -0700, Sean Christopherson wrote:
>> > diff --git a/mm/compaction.c b/mm/compaction.c
>> > index dbc9f86b1934..a3d2b132df52 100644
>> > --- a/mm/compaction.c
>> > +++ b/mm/compaction.c
>> > @@ -1047,6 +1047,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>> > if (!mapping && (folio_ref_count(folio) - 1) > folio_mapcount(folio))
>> > goto isolate_fail_put;
>> >
>> > + /* The mapping truly isn't movable. */
>> > + if (mapping && mapping_unmovable(mapping))
>> > + goto isolate_fail_put;
>> > +
>>
>> I doubt that it is safe to dereference mapping here. I believe the folio
>> can be truncated from under us and the mapping freed with the inode.
>>
>> The folio has to be locked to dereference mapping safely (given that the
>> mapping is still tied to the folio).
>
> There's even a comment to that effect later on in the function:
>
> /*
> * Only pages without mappings or that have a
> * ->migrate_folio callback are possible to migrate
> * without blocking. However, we can be racing with
> * truncation so it's necessary to lock the page
> * to stabilise the mapping as truncation holds
> * the page lock until after the page is removed
> * from the page cache.
> */
>
> (that could be reworded to make it clear how dangerous dereferencing
> ->mapping is without the lock ... and it does need to be changed to say
> "folio lock" instead of "page lock", so ...)
>
> How does this look?
>
> /*
> * Only folios without mappings or that have
> * a ->migrate_folio callback are possible to
> * migrate without blocking. However, we can
> * be racing with truncation, which can free
> * the mapping. Truncation holds the folio lock
> * until after the folio is removed from the page
> * cache so holding it ourselves is sufficient.
> */
Incorporated to my attempt at a fix (posted separately per the requested
process):
https://lore.kernel.org/all/20230901082025.20548-2-vbabka@suse.cz/
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