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 4C33124DCF6; Wed, 15 Apr 2026 08:35:16 +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=1776242117; cv=none; b=ItJYENLIy8cJyGwhWOYoEQcJCtUqYrgzWbC2NXcMcz/pFvn0pH8UBf2BK5Vehx4Ye61kfTUk/wCtawkudK7JIsFiiiV6JMA89o4XXbpzpXnnR2ff5ksoCpSSVUIXSx/oS57h5iMB2aLKU9gjmPUbcCEgAX2vvyR09WcWZ9oDi7g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776242117; c=relaxed/simple; bh=wqq1jxfb037jdTdgNzItw0yzIyAC7MxkmAXbXkvvQVo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=LKJKZi/egOaaKdEUoJtDwvnGFeYohLTx4/G00T6f8iR502yBK0smrLChsjhbINxKZC0mbgGm2eLxgb8xUIO2hfSJ9S6OdME9buWaA4ZROlNr2EYEf76hZtpgz6wKpJHI/hv8c2X6ttZcen0vARKuuCqqjzbiLsp75aSfMviG52Q= 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=Ghv78G0w; 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="Ghv78G0w" 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 0946D4FBA; Wed, 15 Apr 2026 01:35:10 -0700 (PDT) Received: from [10.164.148.46] (MacBook-Pro.blr.arm.com [10.164.148.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D44233F86F; Wed, 15 Apr 2026 01:35:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1776242115; bh=wqq1jxfb037jdTdgNzItw0yzIyAC7MxkmAXbXkvvQVo=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Ghv78G0w0VxXymddvIAQe/GYkOoBW2u+eNE+Z+KEpDpD0TeyUFyfrNwpeGdbDnTBu P96PJ+u86gXFeE2pmbjUPnoZAJV9/JwiSXylZIEjYjyuxFAdsP7Y9BdE6yMi6YT4bV OG+wbMatZn8FPMQ+/fnqx05q8vD6nsBaMpnWKojc= Message-ID: <57b5d0f9-3e56-4f96-9e3a-2f475a78a8eb@arm.com> Date: Wed, 15 Apr 2026 14:05:01 +0530 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: "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, 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> Content-Language: en-US From: Dev Jain In-Reply-To: <2e34b619-085f-4a9c-bb41-bc024fd40dd7@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. > > 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 ... 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. >