linux-kernel - Re: [PATCH] zsmalloc: introduce SG-list based object read API

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Message-ID: <gu5dej7cnw47xr7abuuksqocwt7k25tqad55t3jqwgl2pa4mlr@2kdf5fpy7cbj>
Date: Sat, 17 Jan 2026 12:48:53 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Minchan Kim <minchan@...nel.org>, Nhat Pham <nphamcs@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Brian Geffon <bgeffon@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH] zsmalloc: introduce SG-list based object read API

On (26/01/17 03:23), Yosry Ahmed wrote:
> > > There is a lot of duplication between this and zs_obj_read_begin(). I
> > >  wanted to create a common helper for them both that returns the zpdesc
> > >  and offset, but we cannot do the same on the read end side as the unlock
> > >  needs to happen after kunmap() in zs_obj_read_end().
> > >  
> > >  Putting parts of this code in helpers makes it a bit obscure due to the
> > >  locking rules :/
> > >  
> > >  I wonder if we can drop zs_obj_read_*() and move the spanning logic into
> > >  zram. Looking at zram code, seems like read_from_zspool_raw() and
> > >  read_incompressible_page() just copy the return address, so I think they
> > >  can trivially move to using the SG list helpers and
> > >  memcpy_from_sglist().
> > >  
> > >  The only non-trivial caller is read_compressed_page(), because it passes
> > >  the compressed object to zcomp. So I think we only need to handle the
> > >  linearization there, something like this (completely untested):
> > > 
> > So I was thinking about leaving things as they currently are for this
> > dev cycle, because both zram and zsmalloc have enough of new code queued
> > up. If you don't mind let's remove memcpy() API and convert zram during
> > next cycle (after the upcoming merge window).
> 
> Sure. I think we can do all of it in a single series for the next cycle.
> Add SG interfaces, convert zswap and zram, and remove the old interfaces.

Technically zswap doesn't have to wait for zram, and you can convert
it now, but I don't have any objections to doing both in one series
for the next cycle, sounds good to me.