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[2003:cb:c708:f600:abad:360:c840:33fa]) by smtp.gmail.com with ESMTPSA id b3-20020a5d4d83000000b0020fff0ea0a3sm1457815wru.116.2022.08.26.03.18.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Aug 2022 03:18:58 -0700 (PDT) Message-ID: Date: Fri, 26 Aug 2022 12:18:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages Content-Language: en-US To: alexlzhu@fb.com, linux-mm@kvack.org Cc: willy@infradead.org, hannes@cmpxchg.org, akpm@linux-foundation.org, riel@surriel.com, kernel-team@fb.com, linux-kernel@vger.kernel.org References: <490fcdd204ae129a2e43614a569a1cf4bdde9196.1661461643.git.alexlzhu@fb.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <490fcdd204ae129a2e43614a569a1cf4bdde9196.1661461643.git.alexlzhu@fb.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25.08.22 23:30, alexlzhu@fb.com wrote: > From: Alexander Zhu > > Currently, when /sys/kernel/mm/transparent_hugepage/enabled=always is set > there are a large number of transparent hugepages that are almost entirely > zero filled. This is mentioned in a number of previous patchsets > including: > https://lore.kernel.org/all/20210731063938.1391602-1-yuzhao@google.com/ > https://lore.kernel.org/all/ > 1635422215-99394-1-git-send-email-ningzhang@linux.alibaba.com/ > > Currently, split_huge_page() does not have a way to identify zero filled > pages within the THP. Thus these zero pages get remapped and continue to > create memory waste. In this patch, we identify and free tail pages that > are zero filled in split_huge_page(). In this way, we avoid mapping these > pages back into page table entries and can free up unused memory within > THPs. This is based off the previously mentioned patchset by Yu Zhao. > However, we chose to free zero tail pages whenever they are encountered > instead of only on reclaim or migration. We also add a self test to verify > the RssAnon value to make sure zero pages are not remapped. > Isn't this to some degree splitting the THP (PMDs->PTEs + dissolve compound page) and then letting KSM replace the zero-filled page by the shared zeropage? -- Thanks, David / dhildenb