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 84A922BEC41; Mon, 29 Dec 2025 11:46:51 +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=1767008815; cv=none; b=Ab57ler7g7pgj+e0D+DHnFx26WWFeYqoaMz8DWIAVm/nS/08p8mrW59+kCcA8wzZYwkqW8bATFXZyLII/oNNp/cVJp1iGFEVnmpA7ztjor6dAVg3dY8WzciTGVc8644ZVwknflrIYXUnB7SK3Tsl6uDKO+1U2ASsKbHYy6H/C/M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767008815; c=relaxed/simple; bh=55HigBlQHbzxhusODOnJSc5MeYRZFy/kHhQUkZrIBNE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=s3BZPFmpzFEM+nSTXXWj4S3tJmYEZ4GoISx1pLRSjtrivy+HFV/02MSsmDRm2EGdfhjtO0n6quH6pSlD6u+LmYmRjBxKxO4HbdZ5/6+odB+uOIsM2qylF4F7IDrZEbfb6/GkoRP1lyqtmoQP2IqOZsWd3ECe8d81ql0wGdRFMjM= 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; 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 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 C4A93339; Mon, 29 Dec 2025 03:46:42 -0800 (PST) Received: from [10.57.45.222] (unknown [10.57.45.222]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9303B3F5A1; Mon, 29 Dec 2025 03:46:47 -0800 (PST) Message-ID: <7f0ac47d-1eff-4b79-b260-7812bf3ebc80@arm.com> Date: Mon, 29 Dec 2025 12:46:45 +0100 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/4] selftests/mm: fix faulting-in code in pagemap_ioctl test To: "David Hildenbrand (Red Hat)" , Ryan Roberts , linux-mm@kvack.org, linux-kselftest@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Lorenzo Stoakes , Mark Brown , Shuah Khan , Usama Anjum References: <20251216142633.2401447-1-kevin.brodsky@arm.com> <20251216142633.2401447-4-kevin.brodsky@arm.com> <37210500-6f6e-46ac-ac2f-ac996308590d@arm.com> <6aa47cdb-d2e7-4977-929b-7019b6f991c1@kernel.org> <0575bcf6-c1a3-4ebf-a199-3113758fbdc5@arm.com> From: Kevin Brodsky Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 19/12/2025 09:29, David Hildenbrand (Red Hat) wrote: > On 12/18/25 14:18, Kevin Brodsky wrote: >> On 18/12/2025 09:05, David Hildenbrand (Red Hat) wrote: >>> On 12/16/25 15:56, Ryan Roberts wrote: >>>> On 16/12/2025 14:26, Kevin Brodsky wrote: >>>>> One of the pagemap_ioctl tests attempts to fault in pages by >>>>> memcpy()'ing them to an unused buffer. This probably worked >>>>> originally, but since commit 46036188ea1f ("selftests/mm: build with >>>>> -O2") the compiler is free to optimise away that unused buffer and >>>>> the memcpy() with it. As a result there might not be any resident >>>>> page in the mapping and the test may fail. >>>>> >>>>> We don't need to copy all that memory anyway. Just fault in every >>>>> page by forcing the compiler to read the first byte. >>>>> >>>>> Cc: Usama Anjum >>>>> Signed-off-by: Kevin Brodsky >>>>> --- >>>>>    tools/testing/selftests/mm/pagemap_ioctl.c | 6 +++--- >>>>>    1 file changed, 3 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c >>>>> b/tools/testing/selftests/mm/pagemap_ioctl.c >>>>> index 2cb5441f29c7..67a7a3705604 100644 >>>>> --- a/tools/testing/selftests/mm/pagemap_ioctl.c >>>>> +++ b/tools/testing/selftests/mm/pagemap_ioctl.c >>>>> @@ -1056,7 +1056,6 @@ int sanity_tests(void) >>>>>        struct page_region *vec; >>>>>        char *mem, *fmem; >>>>>        struct stat sbuf; >>>>> -    char *tmp_buf; >>>>>          /* 1. wrong operation */ >>>>>        mem_size = 10 * page_size; >>>>> @@ -1167,8 +1166,9 @@ int sanity_tests(void) >>>>>        if (fmem == MAP_FAILED) >>>>>            ksft_exit_fail_msg("error nomem %d %s\n", errno, >>>>> strerror(errno)); >>>>>    -    tmp_buf = malloc(sbuf.st_size); >>>>> -    memcpy(tmp_buf, fmem, sbuf.st_size); >>>>> +    /* Fault in every page by reading the first byte */ >>>>> +    for (i = 0; i < sbuf.st_size; i += page_size) >>>>> +        (void)*(volatile char *)(fmem + i); >>>> >>>> We have FORCE_READ() in vm_util.h for this. Perhaps that would be >>>> better? >>> >>> Agreed, and if we have multiple patterns where we want to force_read a >>> bigger area, maybe we should provide a helper for that? >> >> I've found just a couple of cases where FORCE_READ() is used for a >> larger area (in hugetlb-madvise.c and split_huge_page_test.c). The step >> size isn't the same in any of these cases though. We could have >> something like fault_area(addr, size, step) but maybe the loops are >> clear enough already? > > Note that even for hugtlb we can read page-per-page, no need to > hugetlb-page-per-hugetlb-page. Not sure if the performance change > would make any real performance difference in this testing code. Fair point. In fact in split_huge_page_test.c we're reading every byte but that's unnecessary. I'll add a helper that reads page-by-page and use that in all 3 cases. - Kevin