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[2003:d8:2f40:b300:53f7:d260:aff4:7256]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a88c8013fesm9865476f8f.38.2025.06.30.01.19.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Jun 2025 01:19:20 -0700 (PDT) Message-ID: Date: Mon, 30 Jun 2025 10:19:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] khugepaged: Reduce race probability between migration and khugepaged To: Dev Jain , Anshuman Khandual , 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> <28bc843d-615f-4848-988b-31a0a2921d14@arm.com> From: David Hildenbrand 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: <28bc843d-615f-4848-988b-31a0a2921d14@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: o3ijRraEVomuWlwJbcfMIWjEqV6OQqE_Cb-ijOAGYQw_1751271562 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 03B2720009 X-Stat-Signature: ct3cyjjwe69a4ix58ooppn916p6mjyw3 X-HE-Tag: 1751271565-655237 X-HE-Meta: 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 /MNgTMcC 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 30.06.25 10:12, Dev Jain wrote: > > On 30/06/25 1:25 pm, 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. > > We already re-attempt migration. See NR_MAX_MIGRATE_PAGES_RETRY and > NR_MAX_MIGRATE_ASYNC_RETRY. Also just before freezing the refcount, > we do a suitable refcount check in folio_migrate_mapping(). So the > race happens after this and folio_ref_freeze() in __folio_migrate_mapping(), > therefore the window for the race is already very small in the migration > path, but large in the khugepaged path. > > >> >>> 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 ? > > IMHO the explanation I have given in the patch description is clear > and succinct: the testcase is failing due to the race. This patch > shortens the race window, and the test on this particular hardware > does not hit the race window again. > >> >>> 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 ? > > SCAN_PMD_MAPPED is used as an indicator to change result to SCAN_SUCCEED > in khugepaged_scan_mm_slot: after the call to collapse_pte_mapped_thp. And, > it is also used in madvise_collapse() to do ++thps which is used to set the > return value of madvise_collapse. So I think this approach will be wrong. But if it already is PMD mapped (just temporarily through a migration entry), isn't this exactly what we want? -- Cheers, David / dhildenb