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[2003:cb:c726:6100:20f2:6848:5b74:ca82]) by smtp.gmail.com with ESMTPSA id w17-20020a05600c475100b004162d06768bsm33906262wmo.21.2024.04.26.01.19.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Apr 2024 01:19:50 -0700 (PDT) Message-ID: Date: Fri, 26 Apr 2024 10:19:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to deferred split list To: Zi Yan , Andrew Morton , linux-mm@kvack.org Cc: "Matthew Wilcox (Oracle)" , Yang Shi , Ryan Roberts , Barry Song <21cnbao@gmail.com>, Lance Yang , linux-kernel@vger.kernel.org References: <20240425211136.486184-1-zi.yan@sent.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: <20240425211136.486184-1-zi.yan@sent.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: A9FEC180003 X-Rspam-User: X-Stat-Signature: 7dkqeu84u3gnbthrtdogir71z3ema8dy X-HE-Tag: 1714119599-815586 X-HE-Meta: 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 VXzdTnel 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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 25.04.24 23:11, Zi Yan wrote: > From: Zi Yan > > In __folio_remove_rmap(), a large folio is added to deferred split list > if any page in a folio loses its final mapping. But it is possible that > the folio is fully unmapped and adding it to deferred split list is > unnecessary. > > For PMD-mapped THPs, that was not really an issue, because removing the > last PMD mapping in the absence of PTE mappings would not have added the > folio to the deferred split queue. > > However, for PTE-mapped THPs, which are now more prominent due to mTHP, > they are always added to the deferred split queue. One side effect > is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be > unintentionally increased, making it look like there are many partially > mapped folios -- although the whole folio is fully unmapped stepwise. > > Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs > where possible starting from commit b06dc281aa99 ("mm/rmap: introduce > folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped > folio is unmapped in one go and can avoid being added to deferred split > list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be > noise when we cannot batch-unmap a complete PTE-mapped folio in one go > -- or where this type of batching is not implemented yet, e.g., migration. > > To avoid the unnecessary addition, folio->_nr_pages_mapped is checked > to tell if the whole folio is unmapped. If the folio is already on > deferred split list, it will be skipped, too. > > Note: commit 98046944a159 ("mm: huge_memory: add the missing > folio_test_pmd_mappable() for THP split statistics") tried to exclude > mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not > fix the above issue. A fully unmapped PTE-mapped order-9 THP was still > added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, > since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside > deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(). > > Signed-off-by: Zi Yan > Reviewed-by: Yang Shi > --- > mm/rmap.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index a7913a454028..220ad8a83589 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, > * page of the folio is unmapped and at least one page > * is still mapped. > */ > - if (folio_test_large(folio) && folio_test_anon(folio)) > - if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) > - deferred_split_folio(folio); > + if (folio_test_large(folio) && folio_test_anon(folio) && > + list_empty(&folio->_deferred_list) && > + ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) || > + (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped))) > + deferred_split_folio(folio); > } > > /* > > base-commit: 66313c66dd90e8711a8b63fc047ddfc69c53636a Reviewed-by: David Hildenbrand But maybe we can really improve the code: diff --git a/mm/rmap.c b/mm/rmap.c index 2608c40dffade..e310b6c4221d7 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, { atomic_t *mapped = &folio->_nr_pages_mapped; int last, nr = 0, nr_pmdmapped = 0; + bool partially_mapped = false; enum node_stat_item idx; __folio_rmap_sanity_checks(folio, page, nr_pages, level); @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, nr++; } } while (page++, --nr_pages > 0); + + partially_mapped = nr && atomic_read(mapped); break; case RMAP_LEVEL_PMD: atomic_dec(&folio->_large_mapcount); @@ -1532,6 +1535,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, nr = 0; } } + partially_mapped = nr < nr_pmdmapped; break; } @@ -1553,9 +1557,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, * page of the folio is unmapped and at least one page * is still mapped. */ - if (folio_test_large(folio) && folio_test_anon(folio)) - if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) - deferred_split_folio(folio); + if (folio_test_large(folio) && folio_test_anon(folio) && + list_empty(&folio->_deferred_list) && partially_mapped) + deferred_split_folio(folio); } /* The compiler should be smart enough to optimize it all -- most likely ;) -- Cheers, David / dhildenb