Re: [PATCH 1/2] mm: zbud: deprecate CONFIG_ZBUD

[Yosry Ahmed]

January 28, 2025 at 2:14 AM, "Johannes Weiner" <hannes@xxxxxxxxxxx> wrote:



> 
> On Mon, Jan 27, 2025 at 11:58:21PM +0000, Yosry Ahmed wrote:
> 
> > 
> > The zbud compressed pages allocator is rarely used, most users use
> > 
> >  zsmalloc. zbud consumes much more memory (only stores 1 or 2 compressed
> > 
> >  pages per physical page). The only advantage of zbud is a marginal
> > 
> >  performance improvement that by no means justify the memory overhead.
> > 
> >  
> > 
> >  Historically, zsmalloc had significantly worse latency than zbud and
> > 
> >  z3fold but offered better memory savings. This is no longer the case as
> > 
> >  shown by a simple recent analysis [1]. In a kernel build test on tmpfs
> > 
> >  in a limited cgroup, zbud 2-3% less time than zsmalloc, but at the cost
> > 
> >  of using ~32% more memory (1.5G vs 1.13G). The tradeoff does not make
> > 
> >  sense for zbud in any practical scenario.
> > 
> >  
> > 
> >  The only alleged advantage of zbud is not having the dependency on
> > 
> >  CONFIG_MMU, but CONFIG_SWAP already depends on CONFIG_MMU anyway, and
> > 
> >  zbud is only used by zswap.
> > 
> >  
> > 
> >  Following in the footsteps of [2], which deprecated z3fold, deprecated
> > 
> >  zbud as planned and remove it in a few cycles if no objections are
> > 
> >  raised from active users.
> > 
> >  
> > 
> >  Rename the user-visible config options so that users with CONFIG_ZBUD=y
> > 
> >  get a new prompt with explanation during make oldconfig. Also, remove
> > 
> >  CONFIG_ZBUD from defconfig.
> > 
> >  
> > 
> >  [1]https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@xxxxxxxxxxxxxx/
> > 
> >  [2]https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@xxxxxxxxxx/
> > 
> >  
> > 
> >  Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
> > 
> 
> Can we just drop it right away?
> 
> The two cycles for z3fold were basically in the "not worth bothering"
> 
> category, since very few downstream production systems rebase that
> 
> frequently.
> 
> zsmalloc has been in use on everything from mobile devices to large
> 
> servers for years. It's been the default since 6.6 (Oct '23) for
> 
> zswap, and the only option for zram from the start.

I certainly do not object, if no one else objects I can do that. We can leave the zpool code around for a bit in case a new allocator shows up tho, just as due diligence.