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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=QQZJDCma; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D7C6E606A31C; Wed, 11 Nov 2026 09:10:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=QQZJDCma Received: by smtp.kernel.org (Postfix) with ESMTPSA id 77FE21F00A05; Wed, 11 Nov 2026 09:10:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794388217; bh=xH9l3vH/C4QDmuSYkhVbjVTYTvtMqxmFM17WSOqQ34E=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=QQZJDCmaKacfblRcagLi02rbg4jcCBwjp9GW5ASiJ8taKOiX+os9eVBIIXUG8CZHs UP9BXvgSxs3gHLnqayo3DnLh2wb3rk/JbGvU6a2Io0b/Qu2Bd3x2ZmAk28LVJCpAk+ ROfQ3iEyQKObZevE0BedWNZrvGK9ayUJyuyshutw3A7iSM3s91J3IqkZRMyQDb6FVn NlCjXqDr0jV4ZgvARvuY0guxenbSBTpwGYlUJhfJv1baYnkjQO1x0ZAR/uT0QPwIee ETXVJTcYUe87V8Vk3RT0Nw7bfpV2NeR7mTFNYEW/olNUeI4L4wYcR9EQ718KHiEL0y hRM9iGghejhlg== Message-ID: Date: Wed, 11 Nov 2026 10:10:07 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 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