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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-Archived-By: 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 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B2A811C000D X-Stat-Signature: kh64rskmiqdhx7bgyxqg59edxhwm7e6f X-Rspam-User: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-HE-Tag: 1793288860-426737 X-HE-Meta: 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 H+y+9S9+ b19bIysx7Ofwq2dshL2/6IvDiZZk/0msL2ghljKIe1+ni8YHKkZ6L+5lT7+yiF3pZq7wAH8MbVDdvHmGvpmWMmvRTuGtCg7XSoMVFnhpB1wNeTksWRo5j9NZXGFsKaJgetGEVbScm1BWXkYoBKColXzKVu/n0OMYuykNS8sz9jMZ7U5vE7Cqs6DbvSO3159wZ5YzjL7Mx/SwUXTXRQDkaUL2vjZp1QMMf+KG0K8OtITgX/YMozY34e2XGgZZt5uMHfkOf 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 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