From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3367C4FD260 for ; Tue, 13 Oct 2026 15:08:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791904097; cv=none; b=MvJK812aYTRrapIOqZ79SNe585w1Qob1OYfe95vaoRvYdq5ERuGbH8ulBOPghjxYqtJ4fF2sxbB80Sacxy1o97cvnCdfVdFj095W0ogCTVTDY7UrcMYqhTpxyfK9gZpsF0S5bdxgIO9/hZQhsk0dgL0dHANkK7GVBB/wUfcuOXU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791904097; c=relaxed/simple; bh=LkghnwpbhbnYsWHlnueJD3Raj490z99etdrvYghWNuY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=eQVR4xJ9JWGssd5Zk/cudiFM2SaHzCTMeYGfQR5FdksGDAI9NgZy2SvUgvbf1JnmlQRyqcJLSw8L8gAdUDwN+l+pygaQuTi38uS/dMhkThbCQhhFEFQ2L5PhUNL4E3VRLGPQS4WjiEpU2ZyrAOQxMcg78Ccr5YRdhXyiqIpwf2U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EqISpLtL; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EqISpLtL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 74C0F1F00893; Tue, 13 Oct 2026 15:08:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791904095; bh=dWq38U/j2zgM+gxF54O0wMCG/5Y2Rb874k7wZE5nvkQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=EqISpLtLkvppqGc8VTu+aPrFFjt9yyE4ZPvcCSZRP89zFv7QXoHLfhf/+NKq6AdHN OZQK0caOH+dZgzVhuWgD2HhGe+4qVpp/VCJsLBjK6E5M5HjLK82AMZT0+5uAl84chT XYtCy44T9m5yAQQvW/QzEIRVkwze7Z5MQiGTTRInHNzbt4AZdqD8DwiYuYa3DJk8k9 6jHeSCapPy5ykiaAvWGonQGk2X7dIGROCJ9+9iMFhRRoSim7J02YEb0N16Aa7QHNvo ogsVq3BBVW4xlykQwdIWLclR0Sn3VahUcT8zFyjetN3/W4jz0ZAn6DA1e67TSnAZX+ lRTm2b553Kimg== Message-ID: Date: Tue, 13 Oct 2026 17:08:08 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 00/14] mm, swap: extendable swap devices (xswap phase I) To: Baoquan He Cc: Klara Modin , Baoquan He , linux-mm@kvack.org, akpm@linux-foundation.org, chrisl@kernel.org, kasong@tencent.com, hannes@cmpxchg.org, nphamcs@gmail.com, baohua@kernel.org, youngjun.park@lge.com, yosry@kernel.org, shikemeng@huaweicloud.com, chengming.zhou@linux.dev, linux-kernel@vger.kernel.org, kunwu.chan@gmail.com References: <20261003003136.750633-1-hebaoquan@kylinos.cn> <565e1744-54ff-4daf-ac14-7b6c49fd3fa2@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/13/26 11:18, Baoquan He wrote: > On 10/12/26 at 03:44pm, David Hildenbrand (Arm) wrote: >> On 10/12/26 11:16, Baoquan He wrote: >>> Hi Klara, >>> >>> ...snip... >>> >>> Thanks a lot for careful reviewing, and sorry for late reply. >>> >>> ...snip... >>> >>> Thanks for the comment. >>> >>> Here xswap.max= is a global boot parameter, while not a per-device or >>> runtie knob. It sets the space space (si->max) of the xswap device >>> at boot and can't change at runtime. So on the two places: the size is >>> a global setting, fixed at boot; the priority is per device, it is in >>> the create interface. They are different things. >>> >>> The default is 2xRAM. From our earlier discussion, this is the largest >>> size we think is sensible, and in practice I don't think it is not >>> reachable in reality. If a system has swapped out 70% of its memory, and >>> even more at 100%, it is already doubtful that it can still function >>> normally. So 2xRAM is an upper bound, not a target. >> >> I remember some overcommitted hypervisor setups where people swapped out >> significantly more -- without any issues. >> >> For example, Windows VMs initialize all VM memory when booting up. Afterwards, >> you might just have plenty of memory that was touched once but will not really >> be used in any near future. >> >> So I would consider some crazy hypervisor scenarios as well where people >> commonly have significant cold memory that is currently not hot (not in the >> working set) but might possibly used in some distant future. > > Hi David, > > Thanks a lot for the valuable input. This is probably the first first-hand > information I have heard about a production environment. And Klara > measured about 160% of RAM in zswap on her RISC-V box, so the 2xRAM default > is reachable in practice, not just in theory. > > So I see 2xRAM as a default, not as a cap. xswap.max= already overrides it. > After your comment and Klara's numbers, I am open to a larger default, or to > a "max" value for the address space. The real limit is still the backing: > the zswap pool plus the block devices. The address space should no longer > be the limiting factor. > > By the way, in Windows-VM case you mentioned, is there a block swap backend, > or is it zswap alone? If it is zswap alone, the knob that matters is > zswap.max_pool_percent, because it sets how much cold memory the pool can > hold. Excellent question: I don't remember all the details. There were talks about over-committing up to 400%, and people played with both zswap and zram to avoid/reduce IO. I assume as a safety net people would always look for an additional swap backend (that's what I remember, but I don't recall all the details). -- Cheers, David