linux-kernel - Re: [PATCH v2 6/7] zram: add bd

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Date:   Wed, 28 Nov 2018 15:30:21 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v2 6/7] zram: add bd_stat statistics

On Tue, 27 Nov 2018 11:07:54 +0900 Minchan Kim <minchan@...nel.org> wrote:

> On Mon, Nov 26, 2018 at 12:58:33PM -0800, Andrew Morton wrote:
> > On Mon, 26 Nov 2018 17:28:12 +0900 Minchan Kim <minchan@...nel.org> wrote:
> > 
> > > +File /sys/block/zram<id>/bd_stat
> > > +
> > > +The stat file represents device's backing device statistics. It consists of
> > > +a single line of text and contains the following stats separated by whitespace:
> > > + bd_count	size of data written in backing device.
> > > +		Unit: pages
> > > + bd_reads	the number of reads from backing device
> > > +		Unit: pages
> > > + bd_writes	the number of writes to backing device
> > > +		Unit: pages
> > 
> > Using `pages' is a bad choice.  And I assume this means that
> > writeback_limit is in pages as well, which is worse.
> > 
> > Page sizes are not constant!  We want userspace which was developed on
> > 4k pagesize to work the same on 64k pagesize.
> > 
> > Arguably, we could require that well-written userspace remember to use
> > getpagesize().  However we have traditionally tried to avoid that by
> > performing the pagesize normalization within the kernel.
> 
> zram works based on page so I used that term but I agree it's rather
> vague. If there is no objection, I will use (Unit: 4K) instead of
> (Unit: pages).

Is that still true if PAGE_SIZE=64k?