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

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Message-ID: <761a2899-6fd9-4bfe-aeaf-23bce0baa0f1@redhat.com>
Date: Tue, 5 Aug 2025 12:47:36 +0200
From: David Hildenbrand <david@...hat.com>
To: SeongJae Park <sj@...nel.org>
Cc: "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>,
 Nhat Pham <nphamcs@...il.com>, 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 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?

-- 
Cheers,

David / dhildenb