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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=FFikltmu; 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 492B2606A31C; Fri, 30 Oct 2026 10:20:40 +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=FFikltmu Received: by smtp.kernel.org (Postfix) with ESMTPSA id 148D51F000C1; Fri, 30 Oct 2026 10:20:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793355640; bh=OrLw9/UJGvMjAGzzPbSW4jaTf//H5JBEzT5oLPpGE4Y=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=FFikltmuWCCwVyRf18ZkqRpgr/N+FXmJlOnXOqQ0ybZVwU6V7yCPqXBcFvzn/aM4r zelRYCas/epyy1RXYdj1HZzPHTLHjO18P3Dp0BevalikTtbxudTMapLoHy8plYMKO6 SuoodU/F/17ARa7nABgQLDo1oNWTDXqeLdQTy1UQvDXFtutv4tUdntpIIZ0omWg3sz 4V6ZLYjTXAWD8lH4WhzR/aWZCStsmXI/GqQyHp8pIgKvdQcLNDsuhsQM1UOvEXr7Sh VbmKFoGVWt5eJ8v5yS6KNEHZOcJmDu9TQByoWxIgQ6JSsiEbAjRb1Mzq1Axr+JQfba RtVvjvUyug49Q== Message-ID: Date: Fri, 30 Oct 2026 11:20:30 +0100 X-Mailing-List: linux-api@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 0/2] mm/memfd: add F_SEAL_PUNCH_HOLE To: Samiullah Khawaja , Andrew Morton , Hugh Dickins Cc: Baolin Wang , Muchun Song , Oscar Salvador , Jeff Layton , Chuck Lever , Alexander Aring , Shuah Khan , Jason Gunthorpe , Gerd Hoffmann , Vivek Kasireddy , Jeff Xu , David Matlack , Pasha Tatashin , linux-mm@kvack.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261026220546.3114241-1-skhawaja@google.com> 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 10/26/26 23:05, Samiullah Khawaja wrote: > This series introduces F_SEAL_PUNCH_HOLE, a new memfd seal that prevents > holes from being punched into a shmem or hugetlbfs memfd while leaving > the file fully writable. > > Background > ========== > > Drivers such as udmabuf and iommufd (via IOMMU_IOAS_MAP_FILE) use > memfd_pin_folios() to pin the folios backing a memfd and hand them to a > device for DMA. Pinning keeps those folios allocated, but it does not > keep them in the file. If userspace later calls > fallocate(FALLOC_FL_PUNCH_HOLE) or madvise(MADV_REMOVE) on the range, > the folios are dropped from the page cache while the device continues > to use them. The pin holder is never notified, and from that point on > the device and the file silently diverge. We have the exact same situation when userspace maps anonymous folios into vfio and then desides to MADV_DONTNEED the pages. Or any other memory backend that is used for such purposes. User space must not zap/punch things that might be pinned elsewhere. That's not specific to memfd, udmabuf, iommufd or anything else. I don't see a reason to make userspace (that should not shoot itself into the foot) a kernel feature it must activate itself to not shoot itself into the foot. -- Cheers, David