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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 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 X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: A3960A0007 X-Stat-Signature: 15r3r9czmkc78wbug9oyf7je8hi7uqkh X-HE-Tag: 1791904096-619048 X-HE-Meta: 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 tyBoFO3G yu2F4OtiflENBZv58CnKBrcQny4iRTd9HFaHUKkKR3KfYCgXdGmwMYZjASWXLpdCrRU6PyKsWy9plgf3sk7c3GJpSAPIl5MIGBi3e6D0wkiKdaAXo/tvv9Tv+K02iPM9+gDpJAjCltYA6dQkffXVphq6/JbjcztDJeAluX32859iTzltwCAUOAEdXREhPLzyufEBADtrm0s5khLNc1URjK72x9Fv5KH8TvujEw9cunY80z7gCzUZMJm19tHEX33XMfIx1Iu5rp67FNf2kNC6BmtHA5UYYWUTPF2ea+/v0548cDFOwQyvj2g/NT+/is3PbZIn7YS9sUKuuEv8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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