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[2003:cb:c706:d800:568a:6ea7:5272:797c]) by smtp.gmail.com with ESMTPSA id gw7-20020a05600c850700b004146e58cc35sm20171248wmb.46.2024.04.15.08.43.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Apr 2024 08:43:52 -0700 (PDT) Message-ID: <81954ad6-5555-41bd-a026-ad13cecc5647@redhat.com> Date: Mon, 15 Apr 2024 17:43:51 +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 , 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: 8bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 8335840006 X-Stat-Signature: 1fkodfxc9bsti6b5531qj9aobtzzudqa X-HE-Tag: 1713195836-251806 X-HE-Meta: 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 02Lg1sT/ z89+iZ5a/xfssrjFUG2b0RzYKousP7dhEldyPPUDRAEoEo2KUHH/IGvWr8OKSf9sM7NjPQFdZIbK4BU0jJd77etjTIoBghQo0D8Us45tNSwBan6DFGL0G5Ffv8p3/GhQnHc4r1iIHIV7ewk3sMMvXcEDVPWVZPJms5ToEv4OVDRqE5WmG0cdUYPCThcybL5nbEw0dCFeymzLOdG8n/gMLL1Co1BrCusjFewTNG18Did4+0p3xObYGcEGlHluAYbQgVipBPElujAmTYG5vwgZnFosSO/OhGCt9phRckZbFeIT6Qai4KUjdWZW1Tz1zQpDUt226BGqpgSf8Be24B/ctfU3+WKYav4Lge4hHYvu4lEOfBAJWZiqY2ylbd0JeMMnrC1yyV2RdjIutuyJKPs2iiPxfwNuCVMjhhbEE8JQskCGxA2q09lCtADc32lbF6HX5JN3Ou/ZLhHgD/kTgZIYxX/hVXQ== 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 22:35, Yang Shi wrote: > On Fri, Apr 12, 2024 at 12:33 PM 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. >> >> (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. > > Maybe we can let rmap remove code not touch deferred split queue in > migration path at all because we know all the PTEs will be converted > to migration entries. And all the migration entries will be converted > back to PTEs regardless of whether try_to_migrate succeeded or not. It's would be just another bandaid I think :/ Maybe a worthwile one, not sure. -- Cheers, David / dhildenb