From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id D45A01DDC37; Tue, 14 Apr 2026 07:31:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776151887; cv=none; b=VDKAVeESEOvVZeR9UDs29YQuUvu17IRxzw4gZNJGraP8yZdGAU0xc96nNTF4DjxZKzrDfIKsQO4E5BV+Uoh2SUeEuW2Sfi9zQYwv6g3yJTXJghir6MTXNvzxq3fTn4l5dFikv2XPvbRFvGfnFVrw3aY1Dsxs9fM7PoU/sTPWAHc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776151887; c=relaxed/simple; bh=1MTJayB2QhMtyhYQJ6koMdeIvmyUr+qdTV/ptmiRC80=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=peKtYUsbLfKWFftz0hAw8Ed9/O5oxgp+ptKhPGAjAYuoLE17s5Z12fHGHu8Y9SvkaxGoS8nNNfcl++n0hQL0hPpWfMDobevWCJ3HLBDNQwBiR1YlnlxlWc5w4dtXeQNx7iL0v7Y2rqCWC6PDIHVouawcuHj2wCJVC/0OgORsRYo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=PW/X3Bs6; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="PW/X3Bs6" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 63C9F22F8; Tue, 14 Apr 2026 00:31:19 -0700 (PDT) Received: from [10.57.89.16] (unknown [10.57.89.16]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 19F6A3F7B4; Tue, 14 Apr 2026 00:31:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1776151885; bh=1MTJayB2QhMtyhYQJ6koMdeIvmyUr+qdTV/ptmiRC80=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=PW/X3Bs6E85EfWr9rSt/SzrOZBqo2Z4jP8P2X7/0vy8jQSQnogPiKh53dCqYE6eZP IEA53U+t5gFcZo1amxZ3m2s8E8I9AkQ0KeDF3rtSV3nXUipRR4XsSPfamLAKKpeV83 bHa0Hn3VYy3p0Hrhmx1hnWspzc6NTgKtxujDJTJ4= Message-ID: Date: Tue, 14 Apr 2026 08:31:21 +0100 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] selftests/mm: Simplify byte pattern checking in mremap_test Content-Language: en-GB To: Dev Jain , "David Hildenbrand (Arm)" , 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, anshuman.khandual@arm.com, Sarthak Sharma References: <20260410143031.148173-1-dev.jain@arm.com> <5297e0da-d8ec-49df-9b32-0d9f907588d6@kernel.org> <8b5544eb-5ec0-4c85-a2da-7a454fa606dc@arm.com> From: Ryan Roberts In-Reply-To: <8b5544eb-5ec0-4c85-a2da-7a454fa606dc@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 14/04/2026 06:09, Dev Jain wrote: > > > On 14/04/26 12:57 am, David Hildenbrand (Arm) wrote: >> On 4/10/26 16:30, 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 are not equal. That made >>> validation expensive in both cases: for "no mismatch" (the common case when >>> mremap is not buggy), it still walked all bytes in C; for "mismatch", it >>> broke out of the loop after printing the mismatch index. >>> >>> Later, my commit 7033c6cc9620 ("selftests/mm: mremap_test: optimize >>> execution time from minutes to seconds using chunkwise memcmp") tried to >>> optimize both cases by using chunk-wise memcmp() and only scanning bytes >>> within a range which has been determined by memcmp as mismatching. >>> >>> But get_sqrt() in that commit is buggy: `high = mid - 1` is applied >>> unconditionally. This makes the speed of checking the mismatch index >>> suboptimal. >> >> So is that the only problem with 7033c6cc9620: the speed? > > Yes. > > I'll explain the algorithm in 7033c6cc9620. > > The problem statement is: given two buffers of equal length n, find the > first mismatch index. > > Algorithm: Divide the buffers into sqrt(n) chunks. Do a memcmp() over > each chunk. If all of them succeed, the buffers are equal, giving the > result in O(sqrt(n)) * t, where t = time taken by memcmp(). > > Otherwise, worst case is that we find the mismatch in the last chunk. > Now brute-force iterate this chunk to find the mismatch. Since chunk > size is sqrt(n), complexity is again > sqrt(n) * t + sqrt(n) = O(sqrt(n)) * t. > > So if get_sqrt() computes a wrong square root, we lose this time > complexity. > > Maybe there is an optimal value of x = #number of chunks of the buffer, > which may not be sqrt(n). > > But given the information we have, a CS course on algorithms will > say this is one of the optimal ways to do it. > >> >>> >>> The mismatch index does not provide useful debugging value here: if >>> validation fails, we know mremap behavior is wrong, and the specific byte >>> offset does not make root-causing easier. >> >> Fully agreed. >> >>> >>> So instead of fixing get_sqrt(), bite the bullet, drop mismatch index >>> scanning and just compare the two byte streams with memcmp(). >> >> How does this affect the execution time of the test? > > I just checked with ./mremap_test -t 0, the variance is very high on my > system. > > In the common case of the test passing: > > before patch, there are multiple sub-length calls to memcmp. > after patch, there is a single full-length call to memcmp. > > So the time should reduce but may not be very distinguishable. My intuition would be the opposite; if you hafve a 4096 byte buffer, I would have thought that a single memcmp would be significantly faster than sqrt(4096) = 64 calls, each over 64 bytes. If you want to keep the common case fast, but also find the first differing offset on failure, I expect you can exploit the fact that the buffers are all page aligned. With some prompting, Codex gave me this: ---8<--- static size_t first_mismatch_offset(const void *buf1, const void *buf2, size_t len) { const uint64_t *ptr1 = buf1; const uint64_t *ptr2 = buf2; size_t word; size_t words = len / sizeof(*ptr1); assert(!((uintptr_t)buf1 & (sizeof(*ptr1) - 1))); assert(!((uintptr_t)buf2 & (sizeof(*ptr2) - 1))); assert(!(len & (sizeof(*ptr1) - 1))); if (!memcmp(buf1, buf2, len)) return len; for (word = 0; word < words; word++) { if (ptr1[word] != ptr2[word]) { const unsigned char *bytes1 = (const unsigned char *)&ptr1[word]; const unsigned char *bytes2 = (const unsigned char *)&ptr2[word]; size_t i; for (i = 0; i < sizeof(*ptr1); i++) { if (bytes1[i] != bytes2[i]) return word * sizeof(*ptr1) + i; } } } return len; } ---8<--- I've not benchmarked it though... Thanks, Ryan > >> >>> >>> Reported-by: Sarthak Sharma >>> Signed-off-by: Dev Jain >> >> Fixes: 7033c6cc9620 ("selftests/mm: mremap_test: optimize execution time >> from minutes to seconds using chunkwise memcmp") >> >> ? > > Not needed. 7033c6cc9620 does not create any incorrectness in the checking > of mismatch index. > >> >>> --- >>> Sorry for sending two patchsets the same day - the problem was made known >>> to me today, and I couldn't help myself but fix it immediately, imagine >>> my embarrassment when I found out that I made a typo in the binary search >>> code which I had been writing consistently throughout college :) >> >> :) >> >>> >>> Applies on mm-unstable. >>> >>> tools/testing/selftests/mm/mremap_test.c | 109 +++-------------------- >>> 1 file changed, 10 insertions(+), 99 deletions(-) >> >> I mean, it certainly looks like a nice cleanup. >> >