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b=QQZJDCmaKacfblRcagLi02rbg4jcCBwjp9GW5ASiJ8taKOiX+os9eVBIIXUG8CZHs UP9BXvgSxs3gHLnqayo3DnLh2wb3rk/JbGvU6a2Io0b/Qu2Bd3x2ZmAk28LVJCpAk+ ROfQ3iEyQKObZevE0BedWNZrvGK9ayUJyuyshutw3A7iSM3s91J3IqkZRMyQDb6FVn NlCjXqDr0jV4ZgvARvuY0guxenbSBTpwGYlUJhfJv1baYnkjQO1x0ZAR/uT0QPwIee ETXVJTcYUe87V8Vk3RT0Nw7bfpV2NeR7mTFNYEW/olNUeI4L4wYcR9EQ718KHiEL0y hRM9iGghejhlg== Message-ID: Date: Wed, 11 Nov 2026 10:10:07 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] ksm: add only_merge_zero_pages to merge only empty anonymous pages To: "Xu Xin (vivo)" Cc: akpm@linux-foundation.org, chengming.zhou@linux.dev, corbet@lwn.net, link@vivo.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, odys.yuan@gmail.com, rppt@kernel.org, surenb@google.com, tujinjiang@huawei.com, vbabka@kernel.org, xiao.wang@vivo.com References: <20261110074312.219847-1-xu.xin@linux.dev> <20261110074451.220524-1-xu.xin@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: ag5o5mg9488zwbyk4n6jmgm3w9utet4t X-Rspamd-Queue-Id: 2FFBE140005 X-Rspam-User: X-Rspamd-Server: rspam08 X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1794388218-907755 X-HE-Meta: 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 DNHCRjRj sYK8F7jR5o6gHCzv0PloaLgyxFnIfoAS4bKdlnbPwHLjhWCvSTM1FV7BA0oEwbmd27ScZdzzn26lnnT6+Bp3v6O1juO+W++43ILe8Pp1trbh9JgZcZC/CWCXFHzNLjf9GgBPJBxuJ0q3NBqzXlwzlf9dD6HQ6JLH3lVQ3vJwMtrCfj8zFdBvI8h8uzhBMS8v0+LygjwV5T8W521aIm3YVuuanG6Uj0jau9Ud9AvDrOMR8a7HbjyidroxfOQ+eQPPK2qeF5UGPpxzMUvUggCEwW7BktDTbadpzGtGgNoxYk340XwZwv3WeS6rlF0B68u1798RFBVb6R/JMPphlW1OicgkWs2YnMTI7Pxdrs+7P093OBK/Abv/qHVrbykDtlVNJbZXURcLIwZfGEK1UbqMEj17j5bXowqBNADKocxu0cyy0GLk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/10/26 08:44, Xu Xin (vivo) wrote: > Add a new sysfs attribute, /sys/kernel/mm/ksm/only_merge_zero_pages, > which, when enabled, makes ksmd merge only empty (zeroed) anonymous > pages with the kernel zero page and skip the stable and unstable trees > entirely for pages that carry data. > > The attribute requires use_zero_pages to be 1, and the two are kept > consistent by symmetric constraints: it cannot be enabled while > use_zero_pages is 0 (-EINVAL), and use_zero_pages cannot be turned > off while it is active (-EBUSY). Both store handlers take > ksm_thread_mutex around their check-and-set so that a concurrent write > to the two attributes cannot leave use_zero_pages=0 and > only_merge_zero_pages=1, a state in which ksmd would scan and checksum > pages but never merge any. The new attribute can be changed at any time > and is only effective for pages merged after the change. > > Benefits in three aspects > ========================= > - Memory saving: > On memory-constrained systems such as Android and embedded devices, > zero-filled anonymous pages are the largest source of duplicate > anonymous memory. They are produced by userspace allocators: the > zeroing of freshly allocated Java objects and buffers in the ART heap > (new byte[]/int[], Bitmap buffers), calloc/memset in native code, and > preallocated media buffers. Merging them with the kernel zero page > frees a significant amount of physical memory, which matters as DRAM > costs keep rising. With this enabled, we measured 100MB–300MB saving > tested with several popular applications on aarch64 (Android). > > - CPU overhead reduction: > Merging only zero pages also reduces the overhead of ksmd. A page that > is not merged with the zero page carries data and can never be merged > with the zero page, so there is no point in searching the stable tree > or inserting it into the unstable tree for it. Skipping those tree > operations for data-bearing pages reduces both the CPU time and the > rhashtable memory footprint of ksmd, which makes zero page merging > practical on low-resource devices where full KSM scanning would be too > expensive. > > - Security: > Finally, this shrinks the KSM side-channel attack surface: only zero > pages are ever merged, so the sharing status of data-bearing pages is > no longer observable through KSM timing side channels. > > Note > ===== > - Suitable for Android, embedded and low-memory devices, where zero-page > deduplication is wanted but full KSM scanning is too costly, and reduced > KSM side-channel exposure is desired. Many handset vendors may prefer > this knob, which can be used as a default fallback policy. > > - Not suitable for server and virtualization workloads containing many > non-zero duplicate pages, where vanilla KSM provides better memory > savings. > > Signed-off-by: Xu Xin (vivo) > Reviewed-by: Huan Yang > Cc: Wang Xiao > --- See my reply to #2. I think the overall feature makes sense, but reusing all of KSM infrastructure just to deduplicate zero-filled pages looks suboptimal. Please investigate how we could selectively reuse some pieces to just scan + dedulicate zero-filled pages, avoiding the whole rmap item stuff etc. -- Cheers, David