linux-kernel - Re: [PATCHv4 4/9] zram: Introduce recompress sysfs knob

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Message-ID: <Y2Wezsw+FQU6KAlq@google.com>
Date:   Sat, 5 Nov 2022 08:22:54 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCHv4 4/9] zram: Introduce recompress sysfs knob

On (22/11/04 10:27), Minchan Kim wrote:
> > objects that are in-compressible (in a way that saves us memory in
> > zsmalloc) by any of the ZRAM's algorithms.
> > 
> > > read_block_state
> > >     zram_algo_idx(zram, index) > 0 ? 'r' : '.');
> > > 
> > > zram_read_from_zpool
> > >     if (zram_algo_idx(zram, idx) != 0)
> > >         idx = 1;
> > 
> > As an idea, maybe we can store everything re-compression related
> > in a dedicated meta field? SKIP flag, algorithm ID, etc.
> > 
> > We don't have too many bits left in ->flags on 32-bit systems. We
> > currently probably need at least 3 bits - one for RECOMP_SKIP and at
> > least 2 for algorithm ID. 2 bits for algorithm ID put us into situation
> > that we can have only 00, 01, 10, 11 as IDs, that is maximum 3 recompress
> > algorithms: 00 is the primary one and the rest are alternative ones.
> > Maximum 3 re-compression algorithms sounds like a reasonable max value to
> > me. Yeah, maybe we can use flags bits for it.
> 
> If possbile, let's go with those three bits into flags since we could
> factor them out into dedicated field, anytime later since it's not ABI.

Ack.