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[2003:cb:c706:d800:568a:6ea7:5272:797c]) by smtp.gmail.com with ESMTPSA id jh2-20020a05600ca08200b00417f700eaeasm14191791wmb.22.2024.04.15.12.19.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Apr 2024 12:19:26 -0700 (PDT) Message-ID: Date: Mon, 15 Apr 2024 21:19:25 +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> <60049ec1-df14-4c3f-b3dd-5d771c2ceac4@redhat.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: rspam12 X-Rspamd-Queue-Id: 52E524000E X-Stat-Signature: 7pcp785nijgucbw96uaze63wamgogwhs X-HE-Tag: 1713208776-581437 X-HE-Meta: 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 GXbrYr12 lbBObD6uArvqbPZuGkQHmJU+7nZNlgQZye9R1HDMOj+RcI4XBNgyUkYsmJnoGpLQZIlbmzwXQtc8FgDIoiaf/RfIQSYYsAuUwyzbZ8WQTvMDTDIgWos797pKihEalHx49B8+PKH0tsgaNL7dKVwgpwvxvtYwiG2nq3qUCmhC2uEZgCa1uPWB0aSK9ikx+HwTDPMqvquRxKIC1bwH4o3Cla+ZuofDbI8LtYpiXP+hmlx4me3abJ7FyiAO3x6G1xdHagD6RKHCTYXIivd1cRFF2JaR9Ql9CtgenjE4Oo5x9+TCYstnuKoD8XCW9lVrSLofCi5OFAatUCUOEOQ0GEvQykSW7PHOspTmjz1ghUkHkIJFK7F3/V24kXZL1sDho6JgZRvtShU/JEkVjhrOHcGeDhYuX2mwMCI8t9e+ulY6Q58oHCe8= 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: >> >> We could have >> * THP_DEFERRED_SPLIT_PAGE >> * THP_UNDO_DEFERRED_SPLIT_PAGE >> * THP_PERFORM_DEFERRED_SPLIT_PAGE >> >> Maybe that would catch more cases (not sure if all, though). Then, you >> could tell how many are still on that list. THP_DEFERRED_SPLIT_PAGE - >> THP_UNDO_DEFERRED_SPLIT_PAGE - THP_PERFORM_DEFERRED_SPLIT_PAGE. >> >> That could give one a clearer picture how deferred split interacts with >> actual splitting (possibly under memory pressure), the whole reason why >> deferred splitting was added after all. > > I'm not quite sure whether there is a solid usecase or not. If we > have, we could consider this. But a simpler counter may be more > preferred. Yes. > >> >>> It may be useful. However the counter is typically used to estimate >>> how many THP are partially unmapped during a period of time. >> >> I'd say it's a bit of an abuse of that counter; well, or interpreting >> something into the counter that that counter never reliably represented. > > It was way more reliable than now. Correct me if I am wrong: now that we only adjust the counter for PMD-sized THP, it is as (un)reliable as it always was. Or was there another unintended change by some of my cleanups or previous patches? > >> >> I can easily write a program that keeps sending your counter to infinity >> simply by triggering that behavior in a loop, so it's all a bit shaky. > > I don't doubt that. But let's get back to reality. The counter used to > stay reasonable and reliable with most real life workloads before > mTHP. There may be over-counting, for example, when unmapping a > PTE-mapped THP which was not on a deferred split queue before. But > such a case is not common for real life workloads because the huge PMD > has to be split by partial unmap for most cases. And the partial unmap > will add the THP to deferred split queue. > > But now a common workload, for example, just process exit, may > probably send the counter to infinity. Agreed, that's stupid. > >> >> Something like Ryans script makes more sense, where you get a clearer >> picture of what's mapped where and how. Because that information can be >> much more valuable than just knowing if it's mapped fully or partially >> (again, relevant for handling with memory waste). > > Ryan's script is very helpful. But the counter has been existing and > used for years, and it is a quick indicator and much easier to monitor > in a large-scale fleet. > > If we think the reliability of the counter is not worth fixing, why > don't we just remove it. No counter is better than a broken counter. Again, is only counting the PMD-sized THPs "fixing" the old use cases? Then it should just stick around. And we can even optimize it for some more cases as proposed in this patch. But there is no easy way to "get it completely right" I'm afraid. -- Cheers, David / dhildenb