linux-kernel - Re: [PATCH 0/3] Allow ZRAM to use any zpool-compatible backend

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Date:   Mon, 14 Oct 2019 09:41:10 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Vitaly Wool <vitalywool@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Streetman <ddstreet@...e.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>,
        Henry Burns <henrywolfeburns@...il.com>,
        Theodore Ts'o <tytso@...nk.org>
Subject: Re: [PATCH 0/3] Allow ZRAM to use any zpool-compatible backend

On Thu, Oct 10, 2019 at 11:04:14PM +0300, Vitaly Wool wrote:
> The coming patchset is a new take on the old issue: ZRAM can currently be used only with zsmalloc even though this may not be the optimal combination for some configurations. The previous (unsuccessful) attempt dates back to 2015 [1] and is notable for the heated discussions it has caused.
> 
> The patchset in [1] had basically the only goal of enabling ZRAM/zbud combo which had a very narrow use case. Things have changed substantially since then, and now, with z3fold used widely as a zswap backend, I, as the z3fold maintainer, am getting requests to re-interate on making it possible to use ZRAM with any zpool-compatible backend, first of all z3fold.
> 
> The preliminary results for this work have been delivered at Linux Plumbers this year [2]. The talk at LPC, though having attracted limited interest, ended in a consensus to continue the work and pursue the goal of decoupling ZRAM from zsmalloc.
> 
> The current patchset has been stress tested on arm64 and x86_64 devices, including the Dell laptop I'm writing this message on now, not to mention several QEmu confugirations.
> 
> [1] https://lkml.org/lkml/2015/9/14/356
> [2] https://linuxplumbersconf.org/event/4/contributions/551/

Please describe what's the usecase in real world, what's the benefit zsmalloc
cannot fulfill by desgin and how it's significant.
I really don't want to make fragmentaion of allocator so we should really see
how zsmalloc cannot achieve things if you are claiming.
Please tell us how to test it so that we could investigate what's the root
cause.