From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f41.google.com (mail-pj1-f41.google.com [209.85.216.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A41913CAA47 for ; Wed, 3 Jun 2026 11:22:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.41 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780485768; cv=none; b=r25ieysxuVuU7OdbXFKp/cv/Y9vQPkq+b2V0bRRxBxDkxWI5kTMc8BA1CHad6vXuAQlMpmEO+wllcJbDDYaEXKlJFWyd6+WmICncy6RBWB7yedZbuioyCpQzmRNxzE77G9/m7He30y3b/gm8d/d2sA4lPqmDrz+M/n3/txd0mXU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780485768; c=relaxed/simple; bh=PBmzY9Mio4AslOoNANTOa4rcMBhtLC4qWVMb7zbJVnk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nAvr6dQUrlxkroFjH4GbFQ7SM8N1FZXvlZ13bcf0CXUnL8DTV5G2mEbPltmQ5ekW2/n0Ca+76zG5wHHkNoMFQym/JYoh8FvniXFmnEUTUB6rl6BmPfMXgq2fYYHgjnNTRknl7CjudiFf5F0I/7HW6LpVUY9IZW0LxOzKyxnwa9E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=ObJVdzjS; arc=none smtp.client-ip=209.85.216.41 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="ObJVdzjS" Received: by mail-pj1-f41.google.com with SMTP id 98e67ed59e1d1-36d98b9aa9aso3250412a91.3 for ; Wed, 03 Jun 2026 04:22:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1780485767; x=1781090567; darn=vger.kernel.org; h=content-transfer-encoding:in-reply-to:from:references:cc:to:subject :user-agent:mime-version:date:message-id:from:to:cc:subject:date :message-id:reply-to; bh=lZSbyRe0Cm9MsW0vFwS1Xzmiqu2z6riqDTx2v5DdObE=; b=ObJVdzjSvhTIWJj7/3c/m8bi+f0C2SYaSqu+iKtGTf3LHbWzpBW9+ojBk6ujAIFmdE mZ/aqeZrFXAH3QeZre45anP8DL3jK7wcSKJoIjORqHQUCX92yNXt7W1KWdCTxePYhGiq k//BKLdn+lHVyvbHVV5kOE4teL7Jx4UPo1Nni8iRi49lUs1zvKTac9Bmb61jMMEwN9Im 61aOHtimiy0N7Y92wlR0dzqxZ8oeVAPKuJzmD4/tPD/JeS4dlxHmiIAuqMQddm5SnU0Q 5sI3xYDXGjsM7o1syNHw8eKE/2/DHCvxMtcgbfaTF/ZRTmlEpj/mzYx5194ZeAzcJod2 2oww== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1780485767; x=1781090567; h=content-transfer-encoding:in-reply-to:from:references:cc:to:subject :user-agent:mime-version:date:message-id:x-gm-gg:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=lZSbyRe0Cm9MsW0vFwS1Xzmiqu2z6riqDTx2v5DdObE=; b=sBDhHYfeRNEcsXQl92DHGhGwQ6GI63WlhTcuUG6/5XQu+MvsoS+aggo9dR9B+R7aHI 2wJ9Mmbwf/1yVcvL5scPkzzOxWrJiQWHFWj9y5ULVSHQCJfzX4sELWZ8s9+JXLINvT0N KikcXTX7YB7pgIahk0GmmMZUTk2dB3Z87O24OZ6Viag4XPcqZN4W/2N/rJ6l9LP1mkei c2HihhP+ePmyYiDBAoFP/Em5nLyY+F3i1TPrwCv1wVfST0Og8Bio6oBIRtGkuc+gMz71 OJWOeOYm6cArUzHTFlYMTc8BZE0Bb1wmi0iDEzsReyvNJbTYJ2weP3sKyV47/AgxrOwF aN8Q== X-Forwarded-Encrypted: i=1; AFNElJ8UC/V5u43pt25+zG1EPUvBYFNtrDzC5lZJNkuHRxW/aamto8ktf3kHMZ1gY4MXVB79Vu9Of0YtOxc=@vger.kernel.org X-Gm-Message-State: AOJu0Yxqy8L/Zf+XlxOeprZfXaVGc5HNISLKolsg2dfb6gDnbgfjF1Hs 5gq47UDnCaTUasGPeb7++I7BNYiaMAWbgVvKtvmidE5/6oySVavxxK6W X-Gm-Gg: Acq92OETDDn+uAOB5ReZMyNGKzt4i9tqQKs/P9vupbTkkhi26IAnRDqn977Pl53Lnrx WHwROeNBsWT7JJbmoDFmv/nqjUeHaDNkMacyY+hlKz6ayfxe5FDyLFvb72xNo+OUgfTk3H0gKBl zts9haoFmERi0oEXL/Jra9uuU1Oua+gZgqoJWTOFAdDEuKFzXxe0kSbGDlHPpX9q0cQKUchrBRl T64bRii0tlCl2yzYItR+1O5yFmKo+DE36GMUYhf4rReFXdtwt4cUc7+BSV3T7U/tSMG04Fs2aRx dT/uBbsiadFORWv1ltGnxYHFJ+BNdrsIkHM9HHAl/xfrOdVUqmhfTogm0Xg699t8o6i4QKNGuTA hJFubMnLrankh0alpCHKYlO3B6w1c+ArEpj2Vvgh0W7go7g0z51WuqDGciYJ8q81NveTxey4mvD FpFzPNMhg9u+ueDVtmkJ4F/GuOP0twXoj/a3WXv+Z2Nc/9Rb1tlvJfOfGP0NXU08fE X-Received: by 2002:a17:90b:4b47:b0:366:5c38:fd61 with SMTP id 98e67ed59e1d1-36e32f1676amr2884302a91.12.1780485766827; Wed, 03 Jun 2026 04:22:46 -0700 (PDT) Received: from [10.125.192.75] ([210.184.73.204]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-36e6b904c88sm802459a91.0.2026.06.03.04.22.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Jun 2026 04:22:46 -0700 (PDT) Message-ID: <6deeaea7-3cd1-4403-29fc-d2dc55c297f8@gmail.com> Date: Wed, 3 Jun 2026 19:22:36 +0800 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.15.0 Subject: Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback To: Yosry Ahmed , Nhat Pham Cc: akpm@linux-foundation.org, tj@kernel.org, hannes@cmpxchg.org, shakeel.butt@linux.dev, mhocko@kernel.org, mkoutny@suse.com, chengming.zhou@linux.dev, muchun.song@linux.dev, roman.gushchin@linux.dev, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Hao Jia References: <20260526114601.67041-1-jiahao.kernel@gmail.com> <20260526114601.67041-3-jiahao.kernel@gmail.com> From: Hao Jia In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2026/5/30 09:40, Yosry Ahmed wrote: > On Fri, May 29, 2026 at 12:58:09PM -0700, Nhat Pham wrote: >> On Tue, May 26, 2026 at 4:46 AM Hao Jia wrote: >>> >>> From: Hao Jia >>> >>> Zswap currently writes back pages to backing swap reactively, triggered >>> either by the shrinker or when the pool reaches its size limit. There is >>> no mechanism to control the amount of writeback for a specific memory >>> cgroup. However, users may want to proactively write back zswap pages, >>> e.g., to free up memory for other applications or to prepare for >>> memory-intensive workloads. >>> >>> Introduce a "zswap_writeback_only" key to the memory.reclaim cgroup >>> interface. When specified, this key bypasses standard memory reclaim >>> and exclusively performs proactive zswap writeback up to the requested >>> budget. If omitted, the default reclaim behavior remains unchanged. >>> >>> Example usage: >>> # Write back 100MB of pages from zswap to the backing swap >>> echo "100M zswap_writeback_only" > memory.reclaim >> >> Hmmm, so this 100MB is the pre-compression size? i.e if this 100 MB >> compresses to 25 MB, then you're only freeing 25 MB? >> >> I'm ok-ish with this, but can you document it? > > That's a good point. I think pre-compressed size doesn't make sense to > be honest. We should care about how much memory we are actually trying > to save by doing writeback here. > > The pre-compressed size is only useful in determining the blast radius, > how many actual pages are going to have slower page faults now. But > then, I don't think there's a reasonable way for userspace to decide > that. > > I understand passing in the compressed size is tricky because we need to > keep track of the size of the compressed pages we end up writing back, > but it should be doable. Agreed. Using pre-compressed size is probably easier to implement. IIRC, interfaces like ZRAM writeback_limit are also calculated using the pre-compressed size. I'll clarify this in the documentation in the next version. > > If we really want pre-compressed size here, then yes we need to make it > very clear, and I vote that we use a separate interface in this case > because memory.reclaim having different meanings for the amount of > memory written to it is extremely counter-intuitive. > Agree. This would indeed break the semantics of memory.reclaim. I will use a separate interface for proactive writeback in the next version. Thanks， Hao >> >> The rest seems solid to me, FWIW. I'll defer to Johannes and Yosry for >> opinions on zswap-only proactive reclaim.