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[2003:cb:c724:4300:430f:1c83:1abc:1d66]) by smtp.gmail.com with ESMTPSA id f10-20020a056000128a00b003436a3cae6dsm2620530wrx.98.2024.04.11.14.15.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Apr 2024 14:15:08 -0700 (PDT) Message-ID: <86722546-1d54-4224-9f31-da4f368cd47e@redhat.com> Date: Thu, 11 Apr 2024 23:15:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list To: Yang Shi Cc: Zi Yan , linux-mm@kvack.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Ryan Roberts , Barry Song <21cnbao@gmail.com>, linux-kernel@vger.kernel.org References: <20240411153232.169560-1-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: 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: 8bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 76FE11C0011 X-Stat-Signature: ae1eowznxgo7iubg6jm1q6edd7bii4u9 X-HE-Tag: 1712870114-837736 X-HE-Meta: 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 U9HzFrDl 5KlWRYKmCpAq5j2HDR5FEKeofSOjl7MJLYxUqP6n2SPTwt/byCrZGob2vJYuqx58Uf2+SEOWz0aY6a3ycQDZ30Dr/icZ2TKGRPGpFN1qcKtCCG7PZp9q0Y7UMyrS1nd8BApqQZBjEfzJ66N+WTe+4tDk2b74X6Kk42QiIi73Xhv9xUmU7W0VadJ0zOYTUFszya3bOcuQmjVjvRSQLBM62w0sUoTZ6w7jS9/HfsWhOeuefk3teNX5dXn+UNQK70HmfKJNxCHIedVv0kX0cSYYg5Bln05Lt+ZYYC2a60BYnZMnCwCXf3OUHdrsj0G/jakIHdyydQLA1m3sVfsa/tOUIs7ZPIqzWyVktH+jhaY7CpEkKgF6tZc/ctxn66FyN6I3+msz07AbfsjQOdc5vU0HK9cZDs4H2Eqli3HhxBlkrOrxjNF1CP4ALPL2PJwO64v3SVahHQ1eww2m7TB+yB/VfxduSKddSWwO2BN5ypXZ/3O5xZIo= 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 11.04.24 21:01, Yang Shi wrote: > On Thu, Apr 11, 2024 at 8:46 AM David Hildenbrand wrote: >> >> On 11.04.24 17:32, 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. It is possible that >>> the folio is unmapped fully, but it is unnecessary to add the folio >>> to deferred split list at all. Fix it by checking folio mapcount before >>> adding a folio to deferred split list. >>> >>> Signed-off-by: Zi Yan >>> --- >>> mm/rmap.c | 9 ++++++--- >>> 1 file changed, 6 insertions(+), 3 deletions(-) >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 2608c40dffad..d599a772e282 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>> enum rmap_level level) >>> { >>> atomic_t *mapped = &folio->_nr_pages_mapped; >>> - int last, nr = 0, nr_pmdmapped = 0; >>> + int last, nr = 0, nr_pmdmapped = 0, mapcount = 0; >>> enum node_stat_item idx; >>> >>> __folio_rmap_sanity_checks(folio, page, nr_pages, level); >>> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>> break; >>> } >>> >>> - atomic_sub(nr_pages, &folio->_large_mapcount); >>> + mapcount = atomic_sub_return(nr_pages, >>> + &folio->_large_mapcount) + 1; >> >> That becomes a new memory barrier on some archs. Rather just re-read it >> below. Re-reading should be fine here. >> >>> do { >>> last = atomic_add_negative(-1, &page->_mapcount); >>> if (last) { >>> @@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>> * is still mapped. >>> */ >>> if (folio_test_large(folio) && folio_test_anon(folio)) >>> - if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) >>> + if ((level == RMAP_LEVEL_PTE && >>> + mapcount != 0) || >>> + (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)) >>> deferred_split_folio(folio); >>> } >> >> But I do wonder if we really care? Usually the folio will simply get >> freed afterwards, where we simply remove it from the list. >> >> If it's pinned, we won't be able to free or reclaim, but it's rather a >> corner case ... >> >> Is it really worth the added code? Not convinced. > > It is actually not only an optimization, but also fixed the broken > thp_deferred_split_page counter in /proc/vmstat. > > The counter actually counted the partially unmapped huge pages (so > they are on deferred split queue), but it counts the fully unmapped > mTHP as well now. For example, when a 64K THP is fully unmapped, the > thp_deferred_split_page is not supposed to get inc'ed, but it does > now. > > The counter is also useful for performance analysis, for example, > whether a workload did a lot of partial unmap or not. So fixing the > counter seems worthy. Zi Yan should have mentioned this in the commit > log. Yes, all that is information that is missing from the patch description. If it's a fix, there should be a "Fixes:". Likely we want to have a folio_large_mapcount() check in the code below. (I yet have to digest the condition where this happens -- can we have an example where we'd use to do the wrong thing and now would do the right thing as well?) -- Cheers, David / dhildenb