From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 05B86372EEF; Wed, 15 Apr 2026 08:50:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776243033; cv=none; b=kppWMH/BCRXqKvGWZUiSgqcjdvD1Mxg4UanZ5A4NP0hbKsEaoHl3U3n+McsmZdOJqnzjdGCXk48y0U1YEH6acqv7656HRKn5asM74SuapgszOPkrwP53CgBOY10fUGLdG5w7UA+N8AMVhou7W39+QElGd+8Y49BkHq9tvCvUwlo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776243033; c=relaxed/simple; bh=EA/NXf/KyrRthCQc5z0PuVqZcc04Ic4HWmatgZmzjVU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=pDSeE/ZbmxukzJJogld7FFPLinKydg6BTv8HDxZ9K40E1kzDTU/3oUyg/GXFcplZ7QR6Ku3zDl6lWfcHsj3QQX01qZzNK4Fa+mHFCNRLz9DCWWJAuZUTlyTfBGHmSa//YcHJ8Q2Q5caz1qt1Ur6Al1DRpE1nLqnP2tju/duWpaw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RyJz40R1; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="RyJz40R1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C8B52C19424; Wed, 15 Apr 2026 08:50:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776243032; bh=EA/NXf/KyrRthCQc5z0PuVqZcc04Ic4HWmatgZmzjVU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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