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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776108477; bh=Hfs+6Y7gcGQJL72F4dYFx5Gznr+OvUZ9Zk/MEnMHgoA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=sq5WgcMYK3l0ZU8RH001hZQzqmJ3HVN+S6B9cNcMMkw4TJetW4VcreAaHi4SRFjAv FQGTb49s+/ZujASfwByx/J+PFsM3VB6fv0ZqUqevLYsYE+iaAHrXiO9lDF5n3MU5PF UsXIi6rs68VbFkxXw5doQQOV6Jg0GpG+deu5k5F6inEItwzivzVttZtCEPbjAVDkE4 NBy1FTG9Fgm/jlPm5Kkt9hMZEXV4JeOJuYYEY/RGqhxDQRUjth6fpxLkHx8f8w51ng mH/3sSu1zvqFwegHgzCTlJBOp1KkM13EttTGrgngClFKw4L1F3tWGyt9x4gT7HVCvZ xbyeMfgJLi11w== Message-ID: <5297e0da-d8ec-49df-9b32-0d9f907588d6@kernel.org> Date: Mon, 13 Apr 2026 21:27:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] 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: <20260410143031.148173-1-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-Stat-Signature: mf4jrysu3ytdyeq7a1q97k14gkbch8qf X-Rspamd-Queue-Id: A6B04C0007 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1776108478-92402 X-HE-Meta: U2FsdGVkX18J5ZlHyj6GZFYS/aul3MHMUS1z9nRKxq5HRmZ2e/1yrkPgy38BDly4HgYecg0PbNhF90Jv0cZ2fD9E9Q6asSo3v6wDzBlj2wK83Yk6Wo4yW+rmyk/77HAP3Hhr2u98UqrXgcVGce2TGUkfKRBPfHrR+Jaiv4KZwLmPv9mudMhonLOGeSySIlCzaCpx5fxNCo1zlBJDcrWUrEJa8n2XfLgDEXndnCjhBW/kkuk1/lxaCuZ5cOmvX+G4mhP+Xz5DXIqRLFOwWYFTgcYwGxWTfuWKCym8tmbysCSfon6X4DLYjbXCIRaSSHRVPRKzGQSIQ8+OLIJ3USA4P8Wu2JDdt4mJOyB3t5zrNu5cFhCdHhVm+eClHfsvRZWSj4FmVIAbMmMrMe7fxVWkJDRPDhAs53dVEnxUQ+HZ1DAJGPnza4JXscpz6qPIq2wjvfVuQGthktFhjWgbzezD5DPI+AuTZv+lyVbTv8RS4iYfum7yWytFBxpjGJuFNqjx6qDCnkZwIHIGanzdpZa80BtJc/vC4AXhvrVapOt4QIDT21lqeKImfgci2qbXMvgLXtqh2A9EwI5U2Vv+MiL8BygCvXyBaiNoguwBSo9ThTgFvZ0WS0oQYykhWol1ejdoIrJgmLNpep6rlqRtSc6D08vAGqrnfo4s7sWU3lGtu48IGpG/ZG6UtfbC3c8XWDxOVwCJvF0ee8Xj/Ifyv+vJEy0pvktPcg6pIqtJxnx/3fvq0k2qQ7l6vkWIFvjYkZ8+WsPnOtjz8pXAzg0OHno6YWTfOV8+B2NQI9TwSC4rZ8H+2eZtjNdLKeba9iJFUkKM2ylYmkbS1TriHuvRApae8GPMmr6VDIxD8u86AJV820HHQ/g/MmR7+Bhcs+b+oSlaCvo7Xd0wHLMYYGFJ2dP6a+hW1ZaFpNOY3fNF07FVa1xrLjwjPmLXzuPzYwy/HJgE/dh+Qh72SJBuSLH1CEK nLj4KWrv mAjBYCmpfYN1sI+urTb9X285lljRvGYN7Ikh1THfiKvdYQjCoBB5VxQGPUceMA6LjF4iYWE9pLRr6VdiepuBG3RFFr6f1UGz5wR1ArpZb9IY4ikA8aQJaplzcedBWUMM3B70AGw1pCJV7tb+/GdZzEmsN352H4CrAkfnEBiu6p7U6nnH0+gmodxuyrhG7MpcskKC/A+3HTA0inNHPrYNKwBk9uYkr6YpWLtAvptogR8YBMWZPgew5H1BMBfNdyYrU+VOryWWoCH9lPdFDXzSZ+C4Sd3qlG2sSV+bk88Sj39SucJPFdnPxXK0a43xtY9HWT3/PnJU26xUHETGndni50ZLRpwzDDsd0ZAPd9yao4v+pqngQ4mk5Pu1t2g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? > > 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? > > 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") ? > --- > 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. -- Cheers, David