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Date:   Wed, 28 Nov 2018 15:35:59 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Joey Pabalinas <joeypabalinas@gmail.com>
Subject: Re: [PATCH v3 5/7] zram: support idle/huge page writeback
Message-Id: <20181128153559.f6645b98a7033b6f6f6b0fbc@linux-foundation.org>
In-Reply-To: <20181127055429.251614-6-minchan@kernel.org>
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        <20181127055429.251614-6-minchan@kernel.org>
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On Tue, 27 Nov 2018 14:54:27 +0900 Minchan Kim <minchan@kernel.org> wrote:

> This patch supports new feature "zram idle/huge page writeback".
> On zram-swap usecase, zram has usually many idle/huge swap pages.
> It's pointless to keep in memory(ie, zram).
> 
> To solve the problem, this feature introduces idle/huge page
> writeback to backing device so the goal is to save more memory
> space on embedded system.
> 
> Normal sequence to use idle/huge page writeback feature is as follows,
> 
> while (1) {
>         # mark allocated zram slot to idle
>         echo all > /sys/block/zram0/idle
>         # leave system working for several hours
>         # Unless there is no access for some blocks on zram,
> 	# they are still IDLE marked pages.
> 
>         echo "idle" > /sys/block/zram0/writeback
> 	or/and
> 	echo "huge" > /sys/block/zram0/writeback
>         # write the IDLE or/and huge marked slot into backing device
> 	# and free the memory.
> }
> 
> By per discussion:
> https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u,
> 
> This patch removes direct incommpressibe page writeback feature
> (d2afd25114f4, zram: write incompressible pages to backing device)
> so we could regard it as regression because incompressible pages
> doesn't go to backing storage automatically. Instead, usre should
> do it via "echo huge" > /sys/block/zram/writeback" manually.

I'm not in any position to determine the regression risk here.

Why is that feature being removed, anyway?

> If we hear some regression, we could restore the function.

Why not do that now?