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b=OylqW79YDrFJPsaXItlffPt6VPHv8BKPYZd34C/sXb9uJu8eh7qEa1AeVaqoNhORF IW2BymuCz9YvLNSt+X5C5fDP/pP0xSiLYJijgEY4Bue8yE8RWo5uu+a0D+5hHIHEl/ yr+N/6Slc/KbYvvgtFS/hW8E9bzEk5joQIRN5e0wEMFQCRFm+R8zaPkiLUBL5pSXe6 9kNyY3DfRaDFxnAkgH7iGtrP8+ejS37vRN3MljtCf86ZJPHC3CoOlw2AwVAJ+OHkL2 8Vs9itBU+Xxy6f10eVxkR+wDrYsd5GDtJmsmmehQ4Cb6s3jvcS6VJGlAwDrNYSHkql vr+UarYIp1dKQ== Message-ID: <2e34b619-085f-4a9c-bb41-bc024fd40dd7@kernel.org> Date: Wed, 15 Apr 2026 09:48:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] selftests/mm: Simplify byte pattern checking in mremap_test To: Dev Jain , akpm@linux-foundation.org, shuah@kernel.org Cc: ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, Sarthak Sharma References: <20260415044509.579428-1-dev.jain@arm.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 X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D50BE14000B X-Stat-Signature: qyu6tnp7zef18g3a7y34pw8ig6x1gew4 X-HE-Tag: 1776239303-696463 X-HE-Meta: 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 Q8gzBtL2 zTQGS4UoBPTar0124QyGYzM4vbDBRjyBKAkHkjOlrdLtPmrhi9+8Eft136d5cMFgE0lssp6gYICdY81AclxYkzzJ+tYaCJVfv4HMCVnwwVXZ44EIlPEM27JKQpozQtW05yRIJBbHtsNQLcaNcOXYDaCF3z0MQFvTHf2tHKXQIYZL4LJY2JMdZGhMvfbQ17e8wSmSiKqf+LBS1gas6CVMYJuIBYXQIj/x5Uax+b0h7FPyU0gtoY5Yqr87HTD/qB78gthfotAnjaIpRYhAcO1PirRqdg5JqZrUZiNCW2r2HkZ78ZVk01lJ9I0P9YnlQPbFZK4I6TxvfJIt7tdBAsGr9qziP9JkDt1qlJer0N1kLVxMPpG6SCoqUVsXyKEfoPgsmP8gMl2d/Bn7Kb9PWcSvn/aO7ZQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/15/26 06:45, Dev Jain wrote: > The original version of mremap_test (7df666253f26: "kselftests: vm: add > mremap tests") validated remapped contents byte-by-byte and printed a > mismatch index in case the bytes streams didn't match. That was rather > inefficient, especially also if the test passed. > > Later, commit 7033c6cc9620 ("selftests/mm: mremap_test: optimize > execution time from minutes to seconds using chunkwise memcmp") used > memcmp() on bigger chunks, to fallback to byte-wise scanning to detect > the problematic index only if it discovered a problem. > > However, the implementation is overly complicated (e.g., get_sqrt() is > currently not optimal) and we don't really have to report the exact > index: whoever debugs the failing test can figure that out. > > Let's simplify by just comparing both byte streams with memcmp() and not > detecting the exact failed index. > > Reported-by: Sarthak Sharma > Tested-by: Sarthak Sharma > Signed-off-by: Dev Jain > --- I'll note something interesting: before 7033c6cc9620, we would check random bytes in the stream. With 7033c6cc9620 we only check the first threshold bytes IIUC. That means, that we are not actually verifying most of the area at all anymore? The whole test options are extremely questionable: $ ./mremap_test --help ./mremap_test: invalid option -- '-' Usage: ./mremap_test [[-t ] [-p ]] -t only validate threshold_mb of the remapped region if 0 is supplied no threshold is used; all tests are run and remapped regions validated fully. The default threshold used is 4MB. -p provide a seed to generate the random pattern for validating the remapped region. Nobody will ever set these parameters, really. And tests that test different things each time they are run are not particularly helpful. We should just remove all that and do something reasonable internally. That is a) Remove all the perf crap (ehm sorry, "advanced tests that don't belong here and that nobody ever runs") from this functional test b) Remove all options from the test. Nobody ever uses them. They are stupid. c) Remove any randomization from the test. There is no need for random patterns, just fill pages with increasing numbers. d) Just always verify the whole regions. Without the rand() magic this will probably be just ... fairly fast? -- Cheers, David