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Date:   Mon, 26 Nov 2018 12:54:46 -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>
Subject: Re: [PATCH v2 7/7] zram: writeback throttle
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In-Reply-To: <20181126082813.81977-8-minchan@kernel.org>
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        <20181126082813.81977-8-minchan@kernel.org>
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On Mon, 26 Nov 2018 17:28:13 +0900 Minchan Kim <minchan@kernel.org> wrote:

> On small memory system, there are lots of write IO so if we use
> flash device as swap, there would be serious flash wearout.
> To overcome the problem, system developers need to design write
> limitation strategy to guarantee flash health for entire product life.
> 
> This patch creates a new konb "writeback_limit" on zram. With that,
> if current writeback IO count(/sys/block/zramX/io_stat) excceds
> the limitation, zram stops further writeback until admin can reset
> the limit.
> 
> +++ b/Documentation/ABI/testing/sysfs-block-zram
> @@ -121,3 +121,12 @@ Contact:	Minchan Kim <minchan@kernel.org>
>  		The bd_stat file is read-only and represents backing device's
>  		statistics (bd_count, bd_reads, bd_writes) in a format
>  		similar to block layer statistics file format.
> +
> +What:		/sys/block/zram<id>/writeback_limit
> +Date:		November 2018
> +Contact:	Minchan Kim <minchan@kernel.org>
> +Description:
> +		The writeback_limit file is read-write and specifies the maximum
> +		amount of writeback ZRAM can do. The limit could be changed
> +		in run time and "0" means disable the limit.
> +		No limit is the initial state.
> diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> index 550bca77d322..41748d52712d 100644
> --- a/Documentation/blockdev/zram.txt
> +++ b/Documentation/blockdev/zram.txt
> @@ -164,6 +164,8 @@ reset             WO    trigger device reset
>  mem_used_max      WO    reset the `mem_used_max' counter (see later)
>  mem_limit         WO    specifies the maximum amount of memory ZRAM can use
>                          to store the compressed data
> +writeback_limit	  WO	specifies the maximum amount of write IO zram can
> +			write out to backing device

Neither the changelog nor the Documentation specify the units of
writeback_limit.  Bytes?  Pages?  Blocks?

This gets so confusing that in many /proc/sys/vm files we actually put
the units into the filenames.