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Thu, 29 Oct 2026 13:38:07 +0000 X-Mizu-Trace-ID: 909f986a67bd46f8 X-Migadu-Flow: FLOW_OUT Message-ID: <2162e5c9-58f4-427f-984d-0099d9b515fc@linux.dev> Date: Thu, 29 Oct 2026 21:37:58 +0800 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: "David Hildenbrand (Arm)" 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> Content-Language: en-US From: Zenghui Yu In-Reply-To: <67c30e4c-3576-4ded-a71f-b49075ed42e3@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: D294D20003 X-Stat-Signature: guoyt95wboe7zz141ibt847x1ujss5kp X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,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-Rspam-User: X-HE-Tag: 1793281088-20041 X-HE-Meta: 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 SuU6TlU5 LzqQlrNWPVLgBYL63gVbP8S7asPvJgs/N9FwT76ZDd8xVmbplmjdmp8ZkawWpgPZYJ3YTXLwFsLdblKTi8ZRkwLRlxMiZ07LxmwXbb4vPuGdWp8FJyoaH1xCv2QBjngrZMfnTAwkuTDqMDuuy8/O6y4103CQZGA9yXjYdTW8oeoqoreiYAMpzRYuDFACck9iH0GQ2lq/lhj587ubQDNRJ+DtUK77i3q5fF2mxgbR2Zahp/VZfCmj8ku7iAff0lR7zU1ezOvemXcAmSGc9AsIlk1Iv6A== 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 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 :) 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. Thanks, Zenghui