linux-kernel - Re: [RFC PATCH v2] mm/zswap: store <PAGE

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: <CAKEwX=MVBJ+Okfsob4Z30_5Y4P8c2DWoS1dYN4dF5dgp4+CexA@mail.gmail.com>
Date: Tue, 5 Aug 2025 09:56:29 -0700
From: Nhat Pham <nphamcs@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: SeongJae Park <sj@...nel.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Michal Hocko <mhocko@...e.com>, 
	Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, kernel-team@...a.com, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Takero Funaki <flintglass@...il.com>
Subject: Re: [RFC PATCH v2] mm/zswap: store <PAGE_SIZE compression failed page as-is

On Tue, Aug 5, 2025 at 3:47 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 05.08.25 02:29, SeongJae Park wrote:
> > When zswap writeback is enabled and it fails compressing a given page,
> > the page is swapped out to the backing swap device.  This behavior
> > breaks the zswap's writeback LRU order, and hence users can experience
> > unexpected latency spikes.  If the page is compressed without failure,
> > but results in a size of PAGE_SIZE, the LRU order is kept, but the
> > decompression overhead for loading the page back on the later access is
> > unnecessary.
> >
> > Keep the LRU order and optimize unnecessary decompression overheads in
> > the cases, by storing the original content in zpool as-is.
>
> Does this have any effect on the movability of the given page? IOW, does
> page migration etc. still work when we store an ordinary page of an
> shmem/anon folio here?

Good question. This depends on the backend allocator of zswap, but the
only backend allocator remaining (zsmalloc) does implement page
migration.

It's why we insisted on using zpool/zsmalloc to handle the
incompressibility case as well:

https://lore.kernel.org/all/CAKEwX=NC65XCkmX1YzivEJtPc+sEJ3pLHUsYhF60QJnk_OtpVw@mail.gmail.com/

>
> --
> Cheers,
>
> David / dhildenb
>