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[2003:cb:c714:e000:d929:2324:97c7:112c]) by smtp.gmail.com with ESMTPSA id j7-20020a5d6187000000b0032dc74c093dsm14303587wru.103.2023.11.16.09.39.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Nov 2023 09:39:02 -0800 (PST) Message-ID: <2d9745ce-e46b-4275-8512-11699f25d921@redhat.com> Date: Thu, 16 Nov 2023 18:39:01 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] ksm: delay the check of splitting compound pages To: xu Cc: akpm@linux-foundation.org, imbrenda@linux.ibm.com, jiang.xuexin@zte.com.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ran.xiaokai@zte.com.cn, wang.yong12@zte.com.cn, xu.xin16@zte.com.cn, yang.yang29@zte.com.cn References: <20231116121715.1974713-1-xu.xin16@zte.com.cn> 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: <20231116121715.1974713-1-xu.xin16@zte.com.cn> 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: A65D2C000F X-Stat-Signature: e7nmyd8gsjmw1d4eyx94m3eodc1yp5qq X-HE-Tag: 1700156346-336576 X-HE-Meta: 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 /5eVY0xX BOBpHR2oxlQNmZj/pjbxResTczzYgG09bjdTb4IIbuT7fAbvRqDikX7JP8XJ6zZM/qcf+6ca2RR2sK3Bf7l2EYyhttorezSjfdOghYFqiZolJpFPDi93qQqqw5ac5SHVHB5xr6mBQeWL1d5mbvYvVM2bdWCWc7H7A5MO62cWVDx7QCwkl/7+ggxcg4RdJOpi7jG0qKR3ISi6+2qfrbzwguXG9xg/oehLe8wDxviVI1Jh931CfkH52i8LLNghkz1zwwjPzE+tJlyqrgF1r9eUwvNRcJnQxwFqnRgq62SUy2wMQqv0O0qE74eynPh988xRl8H7aQY3TYR8aj3eZfAYtIcNLj5v/aiCGvrYfOCGh8j9u2e53m3wVPONk56rMBUw9T/Xt6ZxXYRucw8nCJ/hNDDL9p+qdw1Qxr5xIP/dv/bwwfPmttYiJJAAx/v/tK0PAOqNCQUWom8JPv0HhB/Jyxawj5C+CFpL+3tLf+zahY+kU8Wk= 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 16.11.23 13:17, xu wrote: >>>>> @@ -2229,24 +2229,10 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite >>>>> tree_rmap_item = >>>>> unstable_tree_search_insert(rmap_item, page, &tree_page); >>>>> if (tree_rmap_item) { >>>>> - bool split; >>>>> - >>>>> kpage = try_to_merge_two_pages(rmap_item, page, >>>>> tree_rmap_item, tree_page); >>>>> - /* >>>>> - * If both pages we tried to merge belong to the same compound >>>>> - * page, then we actually ended up increasing the reference >>>>> - * count of the same compound page twice, and split_huge_page >>>>> - * failed. >>>>> - * Here we set a flag if that happened, and we use it later to >>>>> - * try split_huge_page again. Since we call put_page right >>>>> - * afterwards, the reference count will be correct and >>>>> - * split_huge_page should succeed. >>>>> - */ >>>> >>>> I'm curious, why can't we detect that ahead of time and keep only a >>>> single reference? Why do we need the backup code? Anything I am missing? > > Do you mean like this? Let me see if the refcounting here is sane: (a) The caller has a reference on "page" that it will put just after the cmp_and_merge_page() call. (b) unstable_tree_search_insert() obtained a reference to the "tree_page" using get_mergeable_page()->follow_page(). We will put that reference. So indeed, if both references are to the same folio, *we* have two references to the folio and can safely drop one of both. > > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2229,23 +2229,21 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite > tree_rmap_item = > unstable_tree_search_insert(rmap_item, page, &tree_page); > if (tree_rmap_item) { > - bool split; > + bool SameCompound; > + /* > + * If they belongs to the same compound page, its' reference > + * get twice, so need to put_page once to avoid that > + * split_huge_page fails in try_to_merge_two_pages(). > + */ > + if (SameCompound = Is_SameCompound(page, tree_page)) > + put_page(tree_page); > bool same_folio = page_folio(page) == page_folio(tree_page); /* * If both pages belong to the same folio, we are holding two references * to the same large folio: splitting the folio in * try_to_merge_one_page() will fail for that reason. So let's just drop * one reference early. Note that this is only possible if tree_page is * not a KSM page yet. */ if (same_folio) put_page(tree_page); [we could use more folio operations here, but lets KIS] -- Cheers, David / dhildenb