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Date:   Tue, 25 Jul 2023 13:51:55 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paul Moore <paul@paul-moore.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
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        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>
Subject: Re: [RFC PATCH v11 10/29] mm: Add AS_UNMOVABLE to mark mapping as
 completely unmovable
Message-ID: <ZL/Fa4W2Ne9EVxoh@casper.infradead.org>
References: <20230718234512.1690985-1-seanjc@google.com>
 <20230718234512.1690985-11-seanjc@google.com>
 <20230725102403.xywjqlhyqkrzjok6@box.shutemov.name>
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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.
                         */