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[2003:cb:c706:d800:568a:6ea7:5272:797c]) by smtp.gmail.com with ESMTPSA id z4-20020a5d6404000000b00343b09729easm12390017wru.69.2024.04.15.08.13.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Apr 2024 08:13:14 -0700 (PDT) Message-ID: <5b1f986d-e8e2-4ca0-8377-6325076c84fa@redhat.com> Date: Mon, 15 Apr 2024 17:13:13 +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: Zi Yan Cc: linux-mm@kvack.org, Andrew Morton , Matthew Wilcox , Yang Shi , Ryan Roberts , Barry Song <21cnbao@gmail.com>, linux-kernel@vger.kernel.org References: <20240411153232.169560-1-zi.yan@sent.com> <2C698A64-268C-4E43-9EDE-6238B656A391@nvidia.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: 7bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 55604A0006 X-Stat-Signature: o4wr39drekq8dcrwmt8353x5ku8qhjqe X-HE-Tag: 1713193999-557402 X-HE-Meta: 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 Hj6lfR6O 60dNS30NDZQm/+FEIfl1smBmn/a6SwAW6b8nnX2U9/Yqkm2OVvBKukbmNIiBm7S8O2xgakcPpLMbjISmXaOjlRg4sVuxPB0ZJ7UeY0tsGtYkf1z0BhCGtwU35i9poCJM7lIXaTmeXC3aL1jYtB5llr5j38RcJmOELpJTnX67CLC2SOIhRLy4wnjigAUod3Z4hXpJQlNaddQUuxUYt/rMCFjcIvE2hKbJo77zFePjZKZ6XmK4T+p1pHip8xsqTtScXw/3AW0rG3VurilIUDMiqT8+yQHRb9MnguGZfQbzTnJUuX7bBLp9DdSChPBlX+NjvWRQXbmIMZQQT/8PTKaJUBDYIiTlJHoiOco6v 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 12.04.24 23:06, Zi Yan wrote: > On 12 Apr 2024, at 15:32, David Hildenbrand wrote: > >> On 12.04.24 16:35, Zi Yan wrote: >>> On 11 Apr 2024, at 11:46, 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. >>> >>> Would atomic_sub_return_relaxed() work? Originally I was using atomic_read(mapped) >>> below, but to save an atomic op, I chose to read mapcount here. >> >> Some points: >> >> (1) I suggest reading about atomic get/set vs. atomic RMW vs. atomic >> RMW that return a value -- and how they interact with memory barriers. >> Further, how relaxed variants are only optimized on some architectures. >> >> atomic_read() is usually READ_ONCE(), which is just an "ordinary" memory >> access that should not be refetched. Usually cheaper than most other stuff >> that involves atomics. > > I should have checked the actual implementation instead of being fooled > by the name. Will read about it. Thanks. > >> >> (2) We can either use folio_large_mapcount() == 0 or !atomic_read(mapped) >> to figure out if the folio is now completely unmapped. >> >> (3) There is one fundamental issue: if we are not batch-unmapping the whole >> thing, we will still add the folios to the deferred split queue. Migration >> would still do that, or if there are multiple VMAs covering a folio. >> >> (4) We should really avoid making common operations slower only to make >> some unreliable stats less unreliable. >> >> >> We should likely do something like the following, which might even be a bit >> faster in some cases because we avoid a function call in case we unmap >> individual PTEs by checking _deferred_list ahead of time >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 2608c40dffad..356598b3dc3c 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) && >> + (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) && >> + atomic_read(mapped) && >> + data_race(list_empty(&folio->_deferred_list))) > > data_race() might not be needed, as Ryan pointed out[1] Right, I keep getting confused by that. Likely we should add data_race() only if we get actual reports. -- Cheers, David / dhildenb