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[2003:cb:c714:e000:d929:2324:97c7:112c]) by smtp.gmail.com with ESMTPSA id g18-20020a05600c4ed200b004068e09a70bsm39584wmq.31.2023.11.16.10.06.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Nov 2023 10:06:24 -0800 (PST) Message-ID: <664505c5-0782-43b8-aad1-b843616fe893@redhat.com> Date: Thu, 16 Nov 2023 19:06:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] ksm: delay the check of splitting compound pages From: David Hildenbrand 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> <2d9745ce-e46b-4275-8512-11699f25d921@redhat.com> 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: <2d9745ce-e46b-4275-8512-11699f25d921@redhat.com> 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-Stat-Signature: a5k41t1691bynhhruy6xmt49ouxu5erx X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 7189920010 X-Rspam-User: X-HE-Tag: 1700157989-103457 X-HE-Meta: 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 bxBkX/ck 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 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 18:39, David Hildenbrand wrote: > 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] > Looking into the details, that doesn't work unfortunately. try_to_merge_one_page() will call split_huge_page(), but that will only hold a reference to "page", not to "tree page" after the split. Long story short, after split_huge_page() tree_page could get freed because we don't hold a reference to that one anymore. So we most probably cannot do better than the following (untested): diff --git a/mm/ksm.c b/mm/ksm.c index 7efcc68ccc6e..afb079524585 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2226,25 +2226,30 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite if (!err) return; } +retry: tree_rmap_item = unstable_tree_search_insert(rmap_item, page, &tree_page); if (tree_rmap_item) { - bool split; + /* + * 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. + * + * We have to split manually, and lookup the tree_page + * again. Note that we'll only hold a reference to "page" after + * the split. + */ + if (page_folio(page) == page_folio(tree_page)) { + put_page(tree_page); + + if (!trylock_page(page)) + return; + split_huge_page(page); + unlock_page(page); + goto retry; + } 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. - */ - split = PageTransCompound(page) - && compound_head(page) == compound_head(tree_page); put_page(tree_page); if (kpage) { /* @@ -2271,20 +2276,6 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite break_cow(tree_rmap_item); break_cow(rmap_item); } - } else if (split) { - /* - * We are here if we tried to merge two pages and - * failed because they both belonged to the same - * compound page. We will split the page now, but no - * merging will take place. - * We do not want to add the cost of a full lock; if - * the page is locked, it is better to skip it and - * perhaps try again later. - */ - if (!trylock_page(page)) - return; - split_huge_page(page); - unlock_page(page); } } } -- Cheers, David / dhildenb