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[2003:d8:2f40:b300:53f7:d260:aff4:7256]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a892e591d7sm9803295f8f.67.2025.06.30.00.58.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Jun 2025 00:58:52 -0700 (PDT) Message-ID: <79f19625-f5d4-4e53-92c7-c4b34a0a6a98@redhat.com> Date: Mon, 30 Jun 2025 09:58:51 +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] khugepaged: Reduce race probability between migration and khugepaged To: Anshuman Khandual , Dev Jain , akpm@linux-foundation.org Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250630044837.4675-1-dev.jain@arm.com> <786c83e0-d69f-4fa3-a39c-94c4dfc08a20@arm.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: <786c83e0-d69f-4fa3-a39c-94c4dfc08a20@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.06.25 09:55, Anshuman Khandual wrote: > On 30/06/25 10:18 AM, Dev Jain wrote: >> Suppose a folio is under migration, and khugepaged is also trying to >> collapse it. collapse_pte_mapped_thp() will retrieve the folio from the >> page cache via filemap_lock_folio(), thus taking a reference on the folio >> and sleeping on the folio lock, since the lock is held by the migration >> path. Migration will then fail in >> __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of >> such a race happening (leading to migration failure) by bailing out >> if we detect a PMD is marked with a migration entry. > > Could the migration be re-attempted after such failure ? Seems like > the migration failure here is traded for a scan failure instead. > >> >> This fixes the migration-shared-anon-thp testcase failure on Apple M3. > > Could you please provide some more context why this test case was > failing earlier and how does this change here fixes the problem ? > >> >> Note that, this is not a "fix" since it only reduces the chance of >> interference of khugepaged with migration, wherein both the kernel >> functionalities are deemed "best-effort". >>> Signed-off-by: Dev Jain >> --- >> >> This patch was part of >> https://lore.kernel.org/all/20250625055806.82645-1-dev.jain@arm.com/ >> but I have sent it separately on suggestion of Lorenzo, and also because >> I plan to send the first two patches after David Hildenbrand's >> folio_pte_batch series gets merged. >> >> mm/khugepaged.c | 12 ++++++++++-- >> 1 file changed, 10 insertions(+), 2 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 1aa7ca67c756..99977bb9bf6a 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -31,6 +31,7 @@ enum scan_result { >> SCAN_FAIL, >> SCAN_SUCCEED, >> SCAN_PMD_NULL, >> + SCAN_PMD_MIGRATION, >> SCAN_PMD_NONE, >> SCAN_PMD_MAPPED, >> SCAN_EXCEED_NONE_PTE, >> @@ -941,6 +942,8 @@ static inline int check_pmd_state(pmd_t *pmd) >> >> if (pmd_none(pmde)) >> return SCAN_PMD_NONE; >> + if (is_pmd_migration_entry(pmde)) >> + return SCAN_PMD_MIGRATION; >> if (!pmd_present(pmde)) >> return SCAN_PMD_NULL; >> if (pmd_trans_huge(pmde)) >> @@ -1502,9 +1505,12 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, >> !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) >> return SCAN_VMA_CHECK; >> >> - /* Fast check before locking page if already PMD-mapped */ >> + /* >> + * Fast check before locking folio if already PMD-mapped, or if the >> + * folio is under migration >> + */ >> result = find_pmd_or_thp_or_none(mm, haddr, &pmd); >> - if (result == SCAN_PMD_MAPPED) >> + if (result == SCAN_PMD_MAPPED || result == SCAN_PMD_MIGRATION) > Should mapped PMD and migrating PMD be treated equally while scanning ? Wanted to ask the same thing I think: why not simply use SCAN_PMD_MAPPED? After all, the folio is already pmd-mapped, just not using a present entry but (temporarily) using a migration entry. -- Cheers, David / dhildenb