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Fri, 06 Nov 2026 09:11:48 +0000 X-Mizu-Trace-ID: a5d8b693a372ae24 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, willy@infradead.org, jack@suse.cz, ziy@nvidia.com, baolin.wang@linux.alibaba.com, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, kas@kernel.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, peterx@redhat.com, hannes@cmpxchg.org, songliubraving@fb.com, riel@surriel.com, hughd@google.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, jgg@ziepe.ca, kevin.tian@intel.com, joro@8bytes.org, robin.murphy@arm.com, steven.sistare@oracle.com, jhubbard@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, iommu@lists.linux.dev, mason@kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 1/5] mm: limit pagecache folio refcount when mapping pages Date: Fri, 6 Nov 2026 17:11:39 +0800 Message-ID: <20261106091139.67845-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261105-refcount_overflow_stable-v1-1-70b9c655da3e@kernel.org> References: <20261105-refcount_overflow_stable-v1-1-70b9c655da3e@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 3067540002 X-Stat-Signature: wrhjradmy5x85zczshx7c5qd13ot6xyn X-Rspam-User: X-HE-Tag: 1793956310-974338 X-HE-Meta: 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 +882tvpw +h3B/XMFN81cMPjcwFuJ17QjgAOfB4nH7fycdGqlaIWYfqhIPswBnItmNutvzovEs2hJvc6zy0d0BkpwCj3cGXUVpX9oE5D5NDlc/0g6T7sf78EWohNRBCy07SNxz+qBbiYsdthBYoYA7bHPpmrGo+4l/yKh/0Oao/UliojqZdDCnJsQ7ooLn/tEwowVHI1+OLsWsc8pjzRs4EQAVaE8Urx6cyCSNcXbsueZpYXC43DhP79s2qH364mXvXCtlM9RMeCZXF10uJfcytu/0MIjLdhj5e8Ft4bAoirAty6a+SeZZkUmsA51422X+CA+vGk20dIDy+1BJM8AZYQ0= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Thu, Nov 05, 2026 at 02:10:10PM +0100, David Hildenbrand (Arm) wrote: >With large file folios, overflowing the folio mapcount and refcount >became possible. For anon folios it is currently not really an issue: >each large anon folio can only be mapped once into a process. > >That's not really big news, but it's about time that we do something >about it because some modern technology might find ways to abuse it :) > >One of the enablers for that was commit ddc58f27f9ee ("mm: drop tail page >refcounting"), however, file thp support was added later. > >File folios can be mapped multiple times into the same process, allowing >for processes to hold many references to the same folio. > >As a short-term solution, let's introduce two folio refcount limits: >one for ordinary consumers and one for admins. Leave some headroom >before the refcount goes negative, which we really don't want (having >the mapcount go negative is also not expected). > >Start reporting SIGBUS if we run into limits. When restoring mappings >from migration entries, instead zap the pte/pmd entry. We allow for >slightly exceeding the limit to keep things simpler. > >This is far from ideal, but better than all easy alternatives we can >backport comfortably. For now it fixes the big problem: letting >the refcount (+mapcount) go negative and cause trouble. > >This is only a temporary fix, we'll follow up with something better. > >While the refcount checks are not required for order 0 folios, let's >just add them for consistency: each folio_add_file_rmap*() and >folio_dup_file_rmap*() needs a prior check. > >Also, just document -EHWPOISON for copy_present_ptes() while at it. > >Reported-by: Chris Mason >Fixes: 800d8c63b2e9 ("shmem: add huge pages support") >Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") >Fixes: 793917d997df ("mm/readahead: Add large folio readahead") >Cc: stable@vger.kernel.org >Signed-off-by: David Hildenbrand (Arm) >--- > include/linux/mm.h | 33 +++++++++++++++++++++++++++++++++ > mm/filemap.c | 6 ++++++ > mm/huge_memory.c | 24 ++++++++++++++++++++++-- > mm/memory.c | 23 ++++++++++++++++++++--- > mm/migrate.c | 11 +++++++++++ > mm/userfaultfd.c | 4 ++++ > 6 files changed, 96 insertions(+), 5 deletions(-) Nice work! I just went through all folio_add_file_rmap*() and folio_dup_file_rmap*() callers, including callers of set_pte_range(). All covered, looks good to me1 Reviewed-by: Lance Yang