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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=fTqsP2PA; 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 77A07606A332 for ; Thu, 29 Oct 2026 15:47:40 +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=fTqsP2PA Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2F4D1F01816; Thu, 29 Oct 2026 15:47:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793288860; bh=QeBdTykJBJ5zP35pEpq30uLA6gbbI1i43t2r5osi4yI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fTqsP2PAmZAEPesNIu77yJzu2WgeScvhiTz4nuD8H279nZSvE1OezRAHKmk3C7WWz ry5/blQJFAbCPKWI+sJwySf45jzT5wkQMFvYNiJQw2Gk2HPZMrpGS+ZWU9lkExsmxx Y8Zy0x2D/lqP9Yijad7uSZlsbBZHv2Uysvd6aA6Ay/n71gBHzpegm42Ss10IrRKNrM 9KVoPh02+y+wuWPGs7ZbBPvHyUh4fumznfY+Brt4k/NiUpM2Rwj0IMMNAaneRJx/oQ +4/ZdjVOEpzVR9S6ZEU+py0Bwcs8Y6hi1nO5nl6S+AgLivD6kYQpYGitL1O8nrOX8t GT2SC5bE3/BNw== Message-ID: Date: Thu, 29 Oct 2026 16:47:31 +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] selftests/mm: fix uffd move-pmd test crash on 64K base page systems To: Zenghui Yu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, rppt@kernel.org, peterx@redhat.com, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, surenb@google.com, mhocko@suse.com References: <20261011163527.95988-1-zenghui.yu@linux.dev> <67c30e4c-3576-4ded-a71f-b49075ed42e3@kernel.org> <2162e5c9-58f4-427f-984d-0099d9b515fc@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 10/29/26 14:37, Zenghui Yu wrote: > Hi David, > > On 10/13/26 3:39 PM, David Hildenbrand (Arm) wrote: >> On 10/11/26 18:35, Zenghui Yu wrote: >>> From: "Zenghui Yu (Huawei)" >>> >>> uffd_move_pmd_test()/uffd_move_test_common() splits the test area into >>> chunks of chunk_size and moves them one by one. On arm64 with 64K base >>> pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than >>> the whole 32MB (UFFD_TEST_MEM_SIZE) test area: >>> >>> step_count = nr_pages / (chunk_size / page_size) == 0 >>> >>> Because chunk_size > page_size, the test then aligns both areas up to >>> 512MB, which points far outside of the 32MB mappings, and decrements >>> step_count, which underflows to ULONG_MAX. The verification loop then >>> runs away and the first dereference of area_src faults with a SIGSEGV. >>> >>> Skip the test when a single chunk cannot fit into the test area. >> >> That leaves 64k untested, though? Why not adjust the chunk size accordingly? > > Right. Apparently this has never been tested on 64k :) Yeah. And if we start supporting it we should properly test it :) > > I agree that scaling the chunk size is the better approach -- it gives us > actual test coverage on such configuration, instead of bailing out early. > > If anyone is interested in it, please feel free to send a patch. Otherwise > I'll work on it when I get some spare time. I guess no rush on this one. -- Cheers, David