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[2003:cb:c718:5800:c745:7e74:aa59:8dbe]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4366127c4d7sm349400895e9.34.2024.12.30.01.48.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Dec 2024 01:48:19 -0800 (PST) Message-ID: <142a47b6-ac31-465c-917e-7b2e98fddb2f@redhat.com> Date: Mon, 30 Dec 2024 10:48:17 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: All MADV_FREE mTHPs are fully subjected to deferred_split_folio() To: Lance Yang , Barry Song <21cnbao@gmail.com> Cc: Linux-MM , Ryan Roberts , Baolin Wang , Andrew Morton References: 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: ye6kXQYCeCo8pjQaXIetu4E1vxk_8a-D1AYBIRv3KtI_1735552101 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 6365E40006 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: 64tikge5bq8opfzqu3pjcrbgfwude91s X-HE-Tag: 1735552054-986368 X-HE-Meta: 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 OjhJQTnm vQxJAEo8Vd7nlcforzid/JONDfwRCt0zZ237RQnLU0pRwN9sL3sNWg2vdQ7+PrLOyoI+b+BLnbi8jhOuqnD3qCoIN5Bn6Ed+/AOuu2sRHMQ13t6anfM2RMmgkyUzUHxHEbltSiyWWRrn4AInVDYYKWY9COE4xLE96z/wptgzntWdiyhlp3MhXnYWI64LpSTFDo7/b/99Jo2wGCxeXjC2PuQR+YgamiREzQlVMzIgRvcRLelyeNBYoiKePO4wk1trh7DIClI05kI10g+roj3EI9ULW6n6g7NNx91B+MTUZhEdm57T3c3LaDrnMT5SkWey85uuGbJuVBr9L2t2rWp1IRchScdJ/VpTOt8qbh+xhBsxpYDbE/dqNSypZj6+rpF7+Z61T/kpRHly8Og6J5GQn9AxZEciHrZbY9f9L7jEw32oYBCqmbEdAUTJ/QA6NkrMRdqGnD3cy3w71k3kwGo5tf+gzwcd3pAod9VdiSo9CniqHW1M= 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 30.12.24 03:14, Lance Yang wrote: > Hi Barry, > > On Mon, Dec 30, 2024 at 5:13 AM Barry Song <21cnbao@gmail.com> wrote: >> >> Hi Lance, >> >> Along with Ryan, David, Baolin, and anyone else who might be interested, >> >> We’ve noticed an unexpectedly high number of deferred splits. The root >> cause appears to be the changes introduced in commit dce7d10be4bbd3 >> ("mm/madvise: optimize lazyfreeing with mTHP in madvise_free"). Since >> that commit, split_folio is no longer called in mm/madvise.c. Hi, I assume you don't see "deferred splits" at all. You see that a folio was added to the deferred split queue to immediately be removed again as it gets freed. Correct? >> >> However, we are still performing deferred_split_folio for all >> MADV_FREE mTHPs, even for those that are fully aligned with mTHP. >> This happens because we execute a goto discard in >> try_to_unmap_one(), which eventually leads to >> folio_remove_rmap_pte() adding all folios to deferred_split when we >> scan the 1st pte in try_to_unmap_one(). >> >> discard: >> if (unlikely(folio_test_hugetlb(folio))) >> hugetlb_remove_rmap(folio); >> else >> folio_remove_rmap_pte(folio, subpage, vma); Yes, that's kind-of know: we neither do PTE batching during unmap for reclaim nor during unmap for migration. We should add that support. But note, just like I raised earlier in the context of similar to "improved partial-mapped logic in rmap code when batching", we are primarily only pleasing counters here. See below on concurrent shrinker. >> >> This could lead to a race condition with shrinker - deferred_split_scan(). >> The shrinker might call folio_try_get(folio), and while we are scanning >> the second PTE of this folio in try_to_unmap_one(), the entire mTHP >> could be transitioned back to swap-backed because the reference count >> is incremented. >> >> /* >> * The only page refs must be one from isolation >> * plus the rmap(s) (dropped by discard:). >> */ >> if (ref_count == 1 + map_count && >> (!folio_test_dirty(folio) || >> ... >> (vma->vm_flags & VM_DROPPABLE))) { >> dec_mm_counter(mm, MM_ANONPAGES); >> goto discard; >> } Reclaim code holds an additional folio reference and has the folio locked. So I don't think this race can really happen in the way you think it could? Please feel free to correct me if I am wrong. >> >> It also significantly increases contention on ds_queue->split_queue_lock during >> memory reclamation and could potentially introduce other race conditions with >> shrinker as well. > > Good catch! > Call me "skeptical" that this is a big issue, at least regarding the shrinker, but also regarding actual lock contention. :) The issue might be less severe than you think: mostly pleasing counters. But yes, there is room for improvement. >> >> I’m curious if anyone has suggestions for resolving this issue. My >> idea is to use >> folio_remove_rmap_ptes to drop all PTEs at once, rather than >> folio_remove_rmap_pte, >> which processes PTEs one by one for an mTHP. This approach would require some >> changes, such as checking the dirty state of PTEs and performing a TLB >> flush for the >> entire mTHP as a whole in try_to_unmap_one(). > > Yeah, IHMO, it would also be beneficial to reclaim entire mTHPs as a whole > in real-world scenarios where MADV_FREE mTHPs are typically no longer > written ;) We should be implementing folio batching. But it won't be able to cover all cases. In the future, I envision that during reclaim/access bit scanning, we determine whether a folio is partially mapped and add it to the deferred split queue. That's one requirement for getting rid of folio->_nr_page_mapped and reliably detecting all partial mappings, but it also avoids having to messing with this information whenever we (temporarily) unmap only parts of a folio, like we have here. Thanks! -- Cheers, David / dhildenb