linux-kernel - Re: [PATCH v2 7/7] zram: writeback throttle

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Date:   Tue, 27 Nov 2018 11:08:32 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v2 7/7] zram: writeback throttle

On Mon, Nov 26, 2018 at 12:54:46PM -0800, Andrew Morton wrote:
> On Mon, 26 Nov 2018 17:28:13 +0900 Minchan Kim <minchan@...nel.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@...nel.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@...nel.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.
> 

I will use unit as 4K.

Thanks.