linux-kernel - Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

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Date:   Thu, 28 Apr 2022 10:25:59 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

On Wed, Apr 27, 2022 at 03:16:48PM -0700, Minchan Kim wrote:
> On Wed, Apr 27, 2022 at 05:20:29PM -0400, Johannes Weiner wrote:
> > On Wed, Apr 27, 2022 at 01:29:34PM -0700, Minchan Kim wrote:
> > > Hi Johannes,
> > > 
> > > On Wed, Apr 27, 2022 at 12:00:15PM -0400, Johannes Weiner wrote:
> > > > Currently it requires poking at debugfs to figure out the size and
> > > > population of the zswap cache on a host. There are no counters for
> > > > reads and writes against the cache. As a result, it's difficult to
> > > > understand zswap behavior on production systems.
> > > > 
> > > > Print zswap memory consumption and how many pages are zswapped out in
> > > > /proc/meminfo. Count zswapouts and zswapins in /proc/vmstat.
> > > > 
> > > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > > > ---
> > > >  fs/proc/meminfo.c             |  7 +++++++
> > > >  include/linux/swap.h          |  5 +++++
> > > >  include/linux/vm_event_item.h |  4 ++++
> > > >  mm/vmstat.c                   |  4 ++++
> > > >  mm/zswap.c                    | 13 ++++++-------
> > > >  5 files changed, 26 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> > > > index 6fa761c9cc78..6e89f0e2fd20 100644
> > > > --- a/fs/proc/meminfo.c
> > > > +++ b/fs/proc/meminfo.c
> > > > @@ -86,6 +86,13 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
> > > >  
> > > >  	show_val_kb(m, "SwapTotal:      ", i.totalswap);
> > > >  	show_val_kb(m, "SwapFree:       ", i.freeswap);
> > > > +#ifdef CONFIG_ZSWAP
> > > > +	seq_printf(m,  "Zswap:          %8lu kB\n",
> > > > +		   (unsigned long)(zswap_pool_total_size >> 10));
> > > > +	seq_printf(m,  "Zswapped:       %8lu kB\n",
> > > > +		   (unsigned long)atomic_read(&zswap_stored_pages) <<
> > > > +		   (PAGE_SHIFT - 10));
> > > > +#endif
> > > 
> > > I agree it would be very handy to have the memory consumption in meminfo
> > > 
> > > https://lore.kernel.org/all/YYwZXrL3Fu8%2FvLZw@google.com/
> > > 
> > > If we really go this Zswap only metric instead of general term
> > > "Compressed", I'd like to post maybe "Zram:" with same reason
> > > in this patchset. Do you think that's better idea instead of
> > > introducing general term like "Compressed:" or something else?
> > 
> > I'm fine with changing it to Compressed. If somebody cares about a
> > more detailed breakdown, we can add Zswap, Zram subsets as needed.
> 
> Thanks! Please consider ZSWPIN to rename more general term, too.

That doesn't make sense to me.

Zram is a swap backend, its traffic is accounted in PSWPIN/OUT. Zswap
is a writeback cache on top of the swap backend. It has pages
entering, refaulting, and being written back to the swap backend
(PSWPOUT). A zswpout and a zramout are different things.

> > From 8e9e2d6490b7082c41743fbdb9ffd2db4e3ce962 Mon Sep 17 00:00:00 2001
> > From: Johannes Weiner <hannes@...xchg.org>
> > Date: Wed, 27 Apr 2022 17:15:15 -0400
> > Subject: [PATCH] mm: zswap: add basic meminfo and vmstat coverage fix fix
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Just for completeness,

Nacked-by: Johannes Weiner <hannes@...pchg.org>

> > @@ -87,7 +87,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
> >  	show_val_kb(m, "SwapTotal:      ", i.totalswap);
> >  	show_val_kb(m, "SwapFree:       ", i.freeswap);
> >  #ifdef CONFIG_ZSWAP
> > -	seq_printf(m,  "Zswap:          %8lu kB\n",
> > +	seq_printf(m,  "Compressed:     %8lu kB\n",
> >  		   (unsigned long)(zswap_pool_total_size >> 10));
> >  	seq_printf(m,  "Zswapped:       %8lu kB\n",
> >  		   (unsigned long)atomic_read(&zswap_stored_pages) <<
> > -- 
> > 2.35.3
> >