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[79.224.248.168]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a56199aa52sm1772249f8f.36.2025.06.12.04.45.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 12 Jun 2025 04:45:57 -0700 (PDT) Message-ID: <6ceb38ce-c16d-48f2-baca-fef79f8fc058@redhat.com> Date: Thu, 12 Jun 2025 13:45:56 +0200 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 1/2] selftests: khugepaged: fix the shmem collapse failure To: Baolin Wang , akpm@linux-foundation.org Cc: lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shuah@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org References: <42b76dbc-d1a1-4d00-b139-c50e0abf8b0c@linux.alibaba.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <42b76dbc-d1a1-4d00-b139-c50e0abf8b0c@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 12.06.25 13:37, Baolin Wang wrote: > > > On 2025/6/12 18:08, David Hildenbrand wrote: >> On 12.06.25 05:54, Baolin Wang wrote: >>> When running the khugepaged selftest for shmem (./khugepaged all:shmem), >> >> Hmm, this combination is not run automatically through run_tests.sh, >> right? IIUC, it only runs "./khugepaged" which tests anon only ... >> >> Should we add it there? Then I would probably have noticed that myself >> earlier :) > > Yes, see patch 2. Yes, was pleasantly surprised when I found that :) > >>> I encountered the following test failures: >>> " >>> Run test: collapse_full (khugepaged:shmem) >>> Collapse multiple fully populated PTE table.... Fail >>> ... >>> Run test: collapse_single_pte_entry (khugepaged:shmem) >>> Collapse PTE table with single PTE entry present.... Fail >>> ... >>> Run test: collapse_full_of_compound (khugepaged:shmem) >>> Allocate huge page... OK >>> Split huge page leaving single PTE page table full of compound >>> pages... OK >>> Collapse PTE table full of compound pages.... Fail >>> " >>> >>> The reason for the failure is that, it will set MADV_NOHUGEPAGE to >>> prevent >>> khugepaged from continuing to scan shmem VMA after khugepaged finishes >>> scanning in the wait_for_scan() function. Moreover, shmem requires a >>> refault >>> to establish PMD mappings. >>> >>> However, after commit 2b0f922323cc, PMD mappings are prevented if the >>> VMA is >>> set with MADV_NOHUGEPAGE flag, so shmem cannot establish PMD mappings >>> during >>> refault. >> >> Right. It's always problematic when we have some contradicting >> information in the VMA vs. pagecache. >> >>> >>> To fix this issue, we can set the MADV_NOHUGEPAGE flag after the shmem >>> refault. >>> With this fix, the shmem test case passes. >>> >>> Fixes: 2b0f922323cc ("mm: don't install PMD mappings when THPs are >>> disabled by the hw/process/vma") >>> Signed-off-by: Baolin Wang >>> --- >>>   tools/testing/selftests/mm/khugepaged.c | 3 +-- >>>   1 file changed, 1 insertion(+), 2 deletions(-) >>> >>> diff --git a/tools/testing/selftests/mm/khugepaged.c >>> b/tools/testing/selftests/mm/khugepaged.c >>> index 8a4d34cce36b..d462f62d8116 100644 >>> --- a/tools/testing/selftests/mm/khugepaged.c >>> +++ b/tools/testing/selftests/mm/khugepaged.c >>> @@ -561,8 +561,6 @@ static bool wait_for_scan(const char *msg, char >>> *p, int nr_hpages, >>>           usleep(TICK); >>>       } >>> -    madvise(p, nr_hpages * hpage_pmd_size, MADV_NOHUGEPAGE); >>> - >>>       return timeout == -1; >>>   } >>> @@ -585,6 +583,7 @@ static void khugepaged_collapse(const char *msg, >>> char *p, int nr_hpages, >>>       if (ops != &__anon_ops) >>>           ops->fault(p, 0, nr_hpages * hpage_pmd_size); >>> +    madvise(p, nr_hpages * hpage_pmd_size, MADV_NOHUGEPAGE); >>>       if (ops->check_huge(p, expect ? nr_hpages : 0)) >>>           success("OK"); >>>       else >> >> It's a shame we have this weird interface: there is no way we can clear >> VM_HUGEPAGE without setting VM_NOHUGEPAGE :( > > Right. > >> But, do we even care about setting MADV_NOHUGEPAGE at all? IIUC, we'll >> almost immediately later call cleanup_area() where we munmap(), right? > > I tested removing the MADV_NOHUGEPAGE setting, and the khugepaged test > cases all passed. > > However, a potential impact of removing MADV_NOHUGEPAGE is that, > khugepaged might report 'timeout', but check_huge() would still report > 'success' (assuming khugepaged tries to scan the VMA and successfully > collapses it after the timeout). Such test result could be confusing. If we run into the timeout, we return "true" from wait_for_scan(), and in khugepaged_collapse() returns immediately. So we wouldn't issue another check_huge() call in khugepaged_collapse(). Did I miss something? -- Cheers, David / dhildenb