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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CcQjhvE3; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D8B42606A326; Fri, 6 Nov 2026 22:42:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CcQjhvE3 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 461341F0089F; Fri, 6 Nov 2026 22:42:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794004950; bh=r9kJmn9kMaUsIxseYi41rabItJ7bLsBpd/nnhZbIok0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=CcQjhvE3LihkZTfRpe4GaGxaNniXCyBTPnv6vcJX3sBVZApH/MsbvbydsSZ81zFxR NSKD4z99+2BdoVaotgWuzYmPU8/Er/qPZuRp+cR9arHo9nWwaJZsFTQfzKjEiuudWe GfkE+8H9wzKx47fQqMgmGvK5vqVukLysoqIE04NdjiWlapD5RSVtbBjw220dY6v/XR 2S0LJ0WX1+Re7Cb9Gp4E1HSpQnf+y3V6McY5i1ZNbYXDBfYl2js6+Ip5zOiSar4N8O X7W9UXU9bMPkJA1+c2nsq9IZoYHe8ZCDYLfGNeNFjembLaMhHYk4LCktt1+0S25TrV d13VE0RJJZy0Q== Message-ID: <20e54cbc-dd5b-4094-856f-2e63f8e19001@kernel.org> Date: Fri, 6 Nov 2026 22:23:26 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/5] mm: limit pagecache folio refcount when mapping pages To: Lance Yang , azpijr@gmail.com 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, 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 References: <528e3a0e-04fb-427f-88c7-63a7f4bbc505@gmail.com> <20261106100645.77296-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 11:06, Lance Yang wrote: > > On Fri, Nov 06, 2026 at 10:42:17AM +0100, Jose A. Perez de Azpillaga wrote: >> On 11/5/26 2:10 PM, 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. >> in mm-new copy_huge_pmd() now does a folio_dup_file_rmap_pmd() at fork, >> since Konstantin's "mm/huge_memory: copy file THPs at fork like >> copy_present_ptes() does". doesn't that one need the check too? > > Ah, good catch! My local tree didn't have Konstantin's series [1] yet. And > right, that path needs a check too I guess :) So, likely my series will go in first, it fixes a way older problem. I'll tell Andrew to hold that one off and let me handle it. -- Cheers, David