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[2003:cb:c73e:8900:2d8:c9f0:f3fb:d4fd]) by smtp.gmail.com with ESMTPSA id he9-20020a05600c540900b0040a48430837sm13332637wmb.13.2023.11.14.06.43.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Nov 2023 06:43:07 -0800 (PST) Message-ID: Date: Tue, 14 Nov 2023 15:43:06 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] ksm: delay the check of splitting compound pages Content-Language: en-US To: yang.yang29@zte.com.cn, akpm@linux-foundation.org Cc: imbrenda@linux.ibm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ran.xiaokai@zte.com.cn, xu.xin.sc@gmail.com, xu.xin16@zte.com.cn, jiang.xuexin@zte.com.cn, wang.yong12@zte.com.cn References: <202311142036302357580@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: <202311142036302357580@zte.com.cn> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14.11.23 13:36, yang.yang29@zte.com.cn wrote: > From: xu xin > > Background > ========== > When trying to merge two pages, it may fail because the two pages > belongs to the same compound page and split_huge_page fails due to > the incorrect reference to the page. To solve the problem, the commit > 77da2ba0648a4 ("mm/ksm: fix interaction with THP") tries to split the > compound page after try_to_merge_two_pages() fails and put_page in > that case. However it is too early to calculate of the variable 'split' which > indicates whether the two pages belongs to the same compound page. > > What to do > ========== > If try_to_merge_two_pages() succeeds, there is no need to check whether > to splitting compound pages. So we delay the check of splitting compound > pages until try_to_merge_two_pages() fails, which can improve the > processing efficiency of cmp_and_merge_page() a little. > > Signed-off-by: xu xin > Reviewed-by: Yang Yang > --- > mm/ksm.c | 36 ++++++++++++++++++++---------------- > 1 file changed, 20 insertions(+), 16 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 7efcc68ccc6e..c952fe5d9e43 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -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? > - split = PageTransCompound(page) > - && compound_head(page) == compound_head(tree_page); > - put_page(tree_page); > if (kpage) { > + put_page(tree_page); > /* > * The pages were successfully merged: insert new > * node in the stable tree and add both rmap_items. > @@ -2271,7 +2257,25 @@ 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) { > + } else { > + bool split; > + /* > + * 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); Would split = page_folio(page) == page_folio(tree_page); do the trick? No need to mess with compound pages. -- Cheers, David / dhildenb