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Shutemov" To: Matthew Wilcox Cc: Sean Christopherson , Vlastimil Babka , Paolo Bonzini , Marc Zyngier , Oliver Upton , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Andrew Morton , Paul Moore , James Morris , "Serge E. Hallyn" , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, Chao Peng , Fuad Tabba , Jarkko Sakkinen , Yu Zhang , Vishal Annapurve , Ackerley Tng , Maciej Szmigiero , David Hildenbrand , Quentin Perret , Michael Roth , Wang , Liam Merwick , Isaku Yamahata Subject: Re: [RFC PATCH v11 10/29] mm: Add AS_UNMOVABLE to mark mapping as completely unmovable Message-ID: <20230726113617.432nuovswn6odcmx@box> References: <20230718234512.1690985-1-seanjc@google.com> <20230718234512.1690985-11-seanjc@google.com> <20230725102403.xywjqlhyqkrzjok6@box.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: B12922001B X-Rspam-User: X-Stat-Signature: 3qur35pep3ejx88nko4iwusw9caocgzs X-Rspamd-Server: rspam03 X-HE-Tag: 1690371397-715919 X-HE-Meta: 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 u6IbjP8D qDu8f+z9U29rgw6CPkptaFdRtR2uO+vPGHqBOwPiPF+No8Pj21sbRNPiOG/gA8j7zR1M9/yP2NpkCl/JyKyTAEkebBC+SL8AGXSsawe9moyzMe8yBBALhD02CmQgbfXTOonlwR7RY6gVTCueXEQIkt1KN75znJvz12+Jgc/sgTtMcvS2VhGFRaIv3+zjJYnvybk/5oqWeY+msG+OmahjXVMTZFz4HvUIIegoBoKnnavW0pvUd38FlLOoO+g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jul 25, 2023 at 01:51:55PM +0100, 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. > */ > Looks good to me. -- Kiryl Shutsemau / Kirill A. Shutemov