linux-kernel - Re: [PATCH v2] mm/zswap: store <PAGE_SIZE compression failed page as-is

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Message-ID: <CAF8kJuNRTTaFxa7fOJi-4QcQfb2ydvfnQVz1_3fKmo-v__qGsg@mail.gmail.com>
Date: Fri, 15 Aug 2025 15:42:28 -0700
From: Chris Li <chrisl@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: SeongJae Park <sj@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, David Hildenbrand <david@...hat.com>, 
	Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Takero Funaki <flintglass@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2] mm/zswap: store <PAGE_SIZE compression failed page as-is

On Wed, Aug 13, 2025 at 12:42 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Wed, Aug 13, 2025 at 10:07:18AM -0700, Chris Li wrote:
> >
> > If you store uncompressed data in the zpool, zpool has metadata
> > overhead, e.g. allocating the entry->handle for uncompressed pages.
> > If the page is not compressed, another idea is just skip the zpool,
> > store it as a page in the zswap entry as page. We can make a union of
> > entry->handle and entry->incompressble_page. If entry->length ==
> > PAGE_SIZE, use entry->incompressable_page as a page.
>
> The main problem being solved here is to avoid the scenario where the
> incompressible pages are being rotated in LRUs and zswapped multiple
> times and wasting CPU on compressing incompressible pages. SJ's approach
> solves the issue but with some memory overhead (zswap entry). With your
> suggestion and to solve the mentioned issue, we will need to change some
> core parts of reclaim (__remove_mapping()), LRU handling (swap cache
> pages not in LRUs) and refault (putting such pages back in LRU and

No, that is not what I have in mind. I mean store a separate page and
memcpy into it, keep the swap cache folio still reclaimed as normal.
Not much different from a store in the zpool.

The cons really are migration etc. We might have to get it in zsmalloc later.

> should it handle read and write faults differently). So, the cons of
> that approach is more complex code.
>
> Personally I would prefer a simple solution with some overhead over a
> more complicated and error prone solution without overhead. Or maybe you
> have a more simplied approach instead?

Ack, there is more complexity to get the last bit of performance gain.
Maybe in zsmalloc later.

Chris