linux-kernel - Re: [RFC PATCH 2/2] zsmalloc: chain-length configuration should consider other metrics

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Message-ID: <utnvn4oupcmohosp6d3qti54qmdppg2xfw622dq6zz7t66a6ma@2pentzdw3zgk>
Date: Thu, 8 Jan 2026 16:37:11 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Herbert Xu <herbert@...dor.apana.org.au>, Andrew Morton <akpm@...ux-foundation.org>, 
	Nhat Pham <nphamcs@...il.com>, Minchan Kim <minchan@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Brian Geffon <bgeffon@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [RFC PATCH 2/2] zsmalloc: chain-length configuration should
 consider other metrics

On (26/01/07 17:12), Yosry Ahmed wrote:
> On Wed, Jan 07, 2026 at 02:43:42PM +0900, Sergey Senozhatsky wrote:
> > On (26/01/07 05:39), Yosry Ahmed wrote:
> > > > On Tue, Jan 06, 2026 at 04:24:45PM +0000, Yosry Ahmed wrote:
> > [..]
> > > > Adding native SG support to LZO simply means removing the memcpy that
> > > > scomp would otherwise have to do.
> > > 
> > > Yeah the effort to add native support to compressors can be done
> > > separately. For zswap, I think the most common compressors are
> > > actually zstd and LZ4.
> > 
> > I think it's the same for chromeos: lz4 (primary) and zstd
> > (secondary/recompression).  zstd looks very complicated,
> > not sure if we really want to diverge its codebase from
> > the upstream (meta github repo).
> 
> I think there's value in using SG lists even if we do not have support
> for lz4 or zstd. We'll remove the memcpy() logic in zsmalloc and the
> kmap handling memcpy() in zswap if we just pass SG lists from zsmalloc
> to zswap/zram.

Yeah I agree, I guess I can cook something up.

For transition period we can have:
- current "memcpy" API
  for zswap

- SG-list API

I can vmap either on the zram side or have new zsmalloc vmap API
(alongside the memcpy and SG-list APIs).

Once crypto API supports SG-list and algorithms tunables I can
switch zram over from zcomp to crypto API and remove memcpy and
vmap APIs from zsmalloc.