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b=idZ1h2Z7GUr72Wjmzbp18MvqPvCXf8h1CAbzzIn6FYZy2l/51v9A2lzLbeqe2B9OF FiHeCtzRHff/2oOOYOHH1XPfklHBD4ILoM2Heg4hjpkjsYRxOOWsNc4rgcd503jRN5 uCYEEOdHOOYwzBiwgWT/MVxsNHfZnSj6qaJZ0i2DkIxSXqb9C/IUy8pW9lHZBInptm uv6tc2R3EsmvME9JoJE0aWrLIsjUiKO+w2sVYA8zPOPx2OGES1sGwbCJ1RFA8Svetq ULihY1lJvgrnfWYmUV1Yz/hZD1CowkWnmsxA5Y2bj55DK+c0R3UPH7Eod8xQemATVu iK1VlXJvXVhjw== Message-ID: Date: Wed, 18 Mar 2026 09:17:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] selftests/mm: Fix soft-dirty kselftest supported check To: Audra Mitchell Cc: linux-kselftest@vger.kernel.org, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, shuah@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260218184210.206466-1-audra@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 57553100011 X-Stat-Signature: ygrdp6qftsomtwzt446sh55gr38oyqoa X-Rspam-User: X-HE-Tag: 1773821867-37158 X-HE-Meta: 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 7wdUQof3 yPlcOiA4NT2ch1Wjg0keZBh+ECI0cO/zRZHKfqKRrO1Ez7yavCR00ifC6pTHutTkPUGDoM5mV3SRDTmgvPlXD8iafu1QRIDUE2rf7XQBTI9cVjvttRoRQSbOJUWS5TECpQcw9a1nYkALnFMh5+ZUbdUBv/zY/o09DFCIFPy8mL3sF35fFYlASbTEdXYJtEPb8xjj0EJt1sr2TF5K16Sb6Z+zKyp4wpeFfijnPPhTpTnl9yMI6VWw4V0h4Sgf/ikFKOkvZ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/17/26 16:08, Audra Mitchell wrote: > Sorry! I missed this email so never responded! > > On Tue, Feb 24, 2026 at 05:15:14PM +0100, David Hildenbrand (Arm) wrote: >> On 2/18/26 19:42, Audra Mitchell wrote: >>> On architectures with separate user address space, such as s390 or >>> those without an MMU, the call to __access_ok will return true. >> >> Where is this __access_ok() you mention here? Somewhere in >> fs/proc/task_mmu.c? >> >> Where in the soft-dirty test is that triggered? >> >> I'm wondering whether the soft-dirty test should be adjusted, but I did >> not yet understand from where this behavior is triggered. > > The problem arises when we are checking to see what features/categories are > supported. The call chain for the soft-dirty program goes: > > main() > ->test_simple() > ->pagemap_is_softdirty() > ->page_entry_is() > ->pagemap_scan_supported() > ->__pagemap_scan_get_categories() > ->ioctl() > > We enter the kernel with an ioctl, expecting to have an EFAULT returned (see > the comment from pagemap_scan_get_categories(): > > /* Provide an invalid address in order to trigger EFAULT. */ > ret = __pagemap_scan_get_categories(fd, start, (struct page_region *) ~0UL); > > Once we enter the kernel, we will check the arguments passed which includes the > call to access_ok: > > do_pagemap_cmd() > ->do_pagemap_scan() > ->pagemap_scan_get_args() > ->access_ok() > > Here is the path within pagemap_scan_get_args where we expect to fail return > the EFAULT: > > if (arg->vec && !access_ok((void __user *)(long)arg->vec, > size_mul(arg->vec_len, sizeof(struct page_region)))) > return -EFAULT; > > However, if CONFIG_ALTERNATE_USER_ADDRESS_SPACE is enabled or if CONFIG_MMU is > NOT enabled, then we just return true: > > if (IS_ENABLED(CONFIG_ALTERNATE_USER_ADDRESS_SPACE) || > !IS_ENABLED(CONFIG_MMU)) > return true; > > The intent appears to be just getting the categories available to us and > verifying that we have the feature available for testing. However, this corner > case means the soft-dirty test will fail with the following: > Thanks for the information, we should clarify that in the patch description. > # -------------------- > # running ./soft-dirty > # -------------------- > # TAP version 13 > # 1..15 > # Bail out! PAGEMAP_SCAN succeeded unexpectedly > # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0 > # [FAIL] > not ok 1 soft-dirty # exit=1 > # SUMMARY: PASS=0 SKIP=0 FAIL=1 > 1..1 > > Since the intent is just to validate that the features are available to us for > testing, I think we can just modify the check so that we don't fail if we > return 0. > > Let me know what you think, or if you have more questions! What about simply testing for success on a test area, wouldn't that be more reliable and clearer? diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c index a6d4ff7dfdc0..489a8d4d915d 100644 --- a/tools/testing/selftests/mm/vm_util.c +++ b/tools/testing/selftests/mm/vm_util.c @@ -67,21 +67,26 @@ static uint64_t pagemap_scan_get_categories(int fd, char *start) } /* `start` is any valid address. */ -static bool pagemap_scan_supported(int fd, char *start) +static bool pagemap_scan_supported(int fd) { + const size_t pagesize = getpagesize(); static int supported = -1; - int ret; + struct page_region r; + void *test_area; if (supported != -1) return supported; - /* Provide an invalid address in order to trigger EFAULT. */ - ret = __pagemap_scan_get_categories(fd, start, (struct page_region *) ~0UL); - if (ret == 0) - ksft_exit_fail_msg("PAGEMAP_SCAN succeeded unexpectedly\n"); - - supported = errno == EFAULT; - + test_area = mmap(0, pagesize, PROT_READ | PROT_WRITE, + MAP_ANONYMOUS | MAP_PRIVATE, 0, 0); + if (test_area == MAP_FAILED) { + ksft_print_msg("WARN: mmap() failed: %s\n", strerror(errno)); + supported = 0; + } else { + supported = __pagemap_scan_get_categories(fd, test_area, &r) >= 0; + ksft_print_msg("errno: %d\n", errno); + munmap(test_area, pagesize); + } return supported; } @@ -90,7 +95,7 @@ static bool page_entry_is(int fd, char *start, char *desc, { bool m = pagemap_get_entry(fd, start) & pagemap_flags; - if (pagemap_scan_supported(fd, start)) { + if (pagemap_scan_supported(fd)) { bool s = pagemap_scan_get_categories(fd, start) & pagescan_flags; if (m == s) -- 2.43.0 > >> Do we have a Fixes: tag? > > I always hesistate to add a Fixes tag on situations like this since this is a > corner case that was not considered by the original author. If we need a > fixes tag, then it would be: > > Fixes: 600bca580579 ("selftests/mm: check that PAGEMAP_SCAN returns correct categories") Yes, please add that. We nowadays also add proper Fixes tags for tests. -- Cheers, David