linux-kernel - Re: [RFC PATCH v3 7/8] mm/zswap: compressed ram direct integration

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aWWHv-G6cZAiQZJY@gourry-fedora-PF4VCD3F>
Date: Mon, 12 Jan 2026 18:46:07 -0500
From: Gregory Price <gourry@...rry.net>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: linux-mm@...ck.org, cgroups@...r.kernel.org, linux-cxl@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, kernel-team@...a.com,
	longman@...hat.com, tj@...nel.org, hannes@...xchg.org,
	mkoutny@...e.com, corbet@....net, gregkh@...uxfoundation.org,
	rafael@...nel.org, dakr@...nel.org, dave@...olabs.net,
	jonathan.cameron@...wei.com, dave.jiang@...el.com,
	alison.schofield@...el.com, vishal.l.verma@...el.com,
	ira.weiny@...el.com, dan.j.williams@...el.com,
	akpm@...ux-foundation.org, vbabka@...e.cz, surenb@...gle.com,
	mhocko@...e.com, jackmanb@...gle.com, ziy@...dia.com,
	david@...nel.org, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, rppt@...nel.org, axelrasmussen@...gle.com,
	yuanchu@...gle.com, weixugc@...gle.com, yury.norov@...il.com,
	linux@...musvillemoes.dk, rientjes@...gle.com,
	shakeel.butt@...ux.dev, chrisl@...nel.org, kasong@...cent.com,
	shikemeng@...weicloud.com, nphamcs@...il.com, bhe@...hat.com,
	baohua@...nel.org, chengming.zhou@...ux.dev,
	roman.gushchin@...ux.dev, muchun.song@...ux.dev, osalvador@...e.de,
	matthew.brost@...el.com, joshua.hahnjy@...il.com, rakie.kim@...com,
	byungchul@...com, ying.huang@...ux.alibaba.com, apopple@...dia.com,
	cl@...two.org, harry.yoo@...cle.com, zhengqi.arch@...edance.com
Subject: Re: [RFC PATCH v3 7/8] mm/zswap: compressed ram direct integration

On Mon, Jan 12, 2026 at 06:33:16PM -0500, Gregory Price wrote:
> One of the assumptions you have in zswap is that there's some known
> REAL chunk of memory X-GB, and the compression ratio dictates that you
> get to cram more than X-GB of data in there.
> 
> This device flips that on its head.  It lies to the system and says
> there's X-GB, and you can only actually use a fraction of it in the
> worst case - and in the best case you use all of it.
> 
> So in that sense, zswap has "infinite upside" (if you're infinitely
> compressible), whereas this device has "limited upside" (node capacity).
> 
> That changes how you account for things entirely, and that's why
> entry->length always has to be PAGE_SIZE.  Even if the device can tell
> us the real size, i'm not sure how useful that is - you still have to
> charge for an entire `struct page`.
> 
> Time for a good long :think:
> 


hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

now that i have written this out, I wonder if the answer here is for the
zswap_node controller (cxl driver or whatever) to detect high memory
usage and online a new memory block if there is additional capacity
available.

This would look like the swap file increasing in size dynamically,
which is *also* problematic, but it's at least in the same ballpark.

>From a CXL perspective, this would look like a dynamic capacity device.

And the catch would be that we would need the opposite interface:

  zswap.c or cram.c would need an explicit evict interface to allow
  capacity to be offlined if the device needs to shrink the "fake"
  capacity in response to shrinking compression ratios.

Time for a much, much longer :think:

~Gregory