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b=RyJz40R1L75t+ZY06L75VfFbTXy5F4Ogjtdjwak+NBOx6Rhzy+WxuaygGaKd88q3J NmXvwaonSZ+6iCeXRS9JgsqCTWMkME5ldrIgypBfk07MFwQYqik7JiKU7UBZUwR1Ll IOYG7XHFqGVfPW4zen6vhhjAMDN+W5MG6T2I70xiyzdCwpaKVLLh9gsrAXB2fa4cUH CZYrmYrC8lWdXWBqtJ5Iru+WOQhtaRHv7s4QOpu9feEBuMoH83/AbGnMKkQG1LTwLS quCgMJy3//FL1BNx4Djd99fH7601HAqRuZDOARnoK9yXF2s1bfQZx4KmrInLyGMgmB 3/SwXiieJ6kkQ== Message-ID: Date: Wed, 15 Apr 2026 10:50:25 +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> <2e34b619-085f-4a9c-bb41-bc024fd40dd7@kernel.org> <57b5d0f9-3e56-4f96-9e3a-2f475a78a8eb@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: rspam11 X-Rspamd-Queue-Id: C0BBA140005 X-Stat-Signature: grddbkg3ytykad776jmcu4gqpmbxktp3 X-HE-Tag: 1776243033-450494 X-HE-Meta: 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 fY4Z+GCM 93hwAwn6jxx7Lhje3pbKWAkK4e4TVutkeuFiA32ThtvqGJlUSDnSUXriWDS42BWsqn1YFXg4yT0oA5YKozJPHMbyZN/iLnOeVOfBfORrkohtE58cuzNX93eytjFvvUW8WABSwZomEV6S61GMgW96YVyi+vkeLpN8OjlGt5uf0b4gGFmZI8hnBi09G5uoaJkVmQes87h4HprCWQcyYUUAZ5KMAqFjTraA2E2OHcI9RbBUc9ND/zXQVa9tak14vh86yC0QA+Ovo/PqmUlAkCywlMEa1Osx7zyqPGR7EiNffSrV7ioA2ezyAcDGTjpifUA1ieIyoMWcHS3L6w+SLMIuSzz46s9YYmDI1oFnOqmbPK2V+uY5D9FpRMRetrXf+Ndskjzxp//lpBnDbbAc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/15/26 10:35, Dev Jain wrote: > > > On 15/04/26 1:18 pm, David Hildenbrand (Arm) wrote: >> 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. > > Before 7033c6cc9620, the block of code was: > > /* Verify byte pattern after remapping */ > srand(pattern_seed); > for (t = 0; t < threshold; t++) { > char c = (char) rand(); > > if (((char *) dest_addr)[t] != c) { > ksft_print_msg("Data after remap doesn't match at offset %llu\n", > t); > ksft_print_msg("Expected: %#x\t Got: %#x\n", c & 0xff, > ((char *) dest_addr)[t] & 0xff); > ret = -1; > goto clean_up_dest; > } > } > > which is still checking the first threshold bytes only. Note that > pattern_seed remains constant at runtime, so 7033c6cc9620 just replaces > this with a buffer filled with the rand() stream. Ah, thanks for clarifying, I got lost in the changes. So to your patch here, definitely Acked-by: David Hildenbrand (Arm) > >> >> 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 > > Hmm... perhaps this is useful, we can keep this by default so we can > detect if a bug comes up in PMD/PUD mremap? If the test takes too long > we know we have messed up something there. Although "test taking too > long" is not a nice way to know that there is a bug ... Exactly. And we should strive for tests in selftests/mm that just work (tm). Any other useful (benchmarking) tools can go to tools/mm. > > and test won't even take long perhaps since memcmp on 1G will be fairly > fast. In case we mess up PMD/PUD mremap real bug reports will come sooner > than anyone detecting this from mremap_test. > > So I'll remove this. > >> >> b) Remove all options from the test. Nobody ever uses them. They are >> stupid. > > Agreed. > >> >> c) Remove any randomization from the test. There is no need for random >> patterns, just fill pages with increasing numbers. > > Agreed. > >> >> d) Just always verify the whole regions. Without the rand() magic this >> will probably be just ... fairly fast? > > Yeah we are doing a simple memcmp() so it is fine. > > I'll implement these changes. Thanks a bunch! -- Cheers, David