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Date:   Tue, 7 Apr 2020 13:24:12 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Jan Kara <jack@suse.cz>
Cc:     NeilBrown <neilb@suse.de>,
        Trond Myklebust <trondmy@hammerspace.com>,
        "Anna.Schumaker@Netapp.com" <Anna.Schumaker@netapp.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-nfs@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK
 instead.
Message-ID: <20200407112412.GO18914@dhcp22.suse.cz>
References: <draft-87d08kw57p.fsf@notabene.neil.brown.name>
 <878sj8w55y.fsf@notabene.neil.brown.name>
 <20200407102515.GB9482@quack2.suse.cz>
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On Tue 07-04-20 12:25:15, Jan Kara wrote:
> On Tue 07-04-20 09:44:25, NeilBrown wrote:
> > @@ -5283,7 +5282,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> >  			" anon_thp: %lukB"
> >  #endif
> >  			" writeback_tmp:%lukB"
> > -			" unstable:%lukB"
> > +			" unstable:0kB"
> >  			" all_unreclaimable? %s"
> >  			"\n",
> >  			pgdat->node_id,
> > @@ -5305,7 +5304,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> >  			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
> >  #endif
> >  			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> > -			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
> >  			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
> >  				"yes" : "no");
> >  	}
> 
> These are just page allocator splats on OOM. I don't think preserving
> 'unstable' in these reports is needed.

YOu are right and the less we dump from this path the better. I could
have noticed.

> > @@ -1707,8 +1706,16 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
> >  static void *vmstat_next(struct seq_file *m, void *arg, loff_t *pos)
> >  {
> >  	(*pos)++;
> > -	if (*pos >= NR_VMSTAT_ITEMS)
> > +	if (*pos >= NR_VMSTAT_ITEMS) {
> > +		/*
> > +		 * Deprecated counters which are no longer represented
> > +		 * in vmstat arrays. We just lie about them to be always
> > +		 * 0 to not break userspace which might expect them in
> > +		 * the output.
> > +		 */
> > +		seq_puts(m, "nr_unstable 0");
> >  		return NULL;
> > +	}
> >  	return (unsigned long *)m->private + *pos;
> >  }
> 
> Umm, how is this supposed to work? vmstat_next() should return next element
> of the sequence, not fill anything into seq_file - that's the job of
> vmstat_show(). Looking at seq_read() implementation it may actually end up
> working fine but I wouldn't really bet much on it especially in corner
> cases like when we are just about to fill the user buffer and then need to
> restart reading close to an end of vmstat file or so.

Well, I have to confess I haven't really tested this myself but the
logic was to have this output close to NR_VMSTAT_ITEMS break out of
the counters loop.

> Michal, won't it be cleaner to have NR_VM_DEPRECATED_ITEMS included in
> NR_VMSTAT_ITEMS, have names of these items in vmstat_text, and just set
> appropriate number of 0 entries at the end of the array generated in
> vmstat_start() and be done with it? That seems conceptually simpler and the
> overhead is minimal.

Yes, that would be much nicer, albeit more code.  So I believe you meant
something like this?

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..a18611197bea 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -237,7 +237,6 @@ enum node_stat_item {
 	NR_FILE_THPS,
 	NR_FILE_PMDMAPPED,
 	NR_ANON_THPS,
-	NR_UNSTABLE_NFS,	/* NFS unstable pages */
 	NR_VMSCAN_WRITE,
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..992e162f1886 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
 	"nr_file_hugepages",
 	"nr_file_pmdmapped",
 	"nr_anon_transparent_hugepages",
-	"nr_unstable",
 	"nr_vmscan_write",
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
@@ -1293,9 +1292,13 @@ const char * const vmstat_text[] = {
 	"swap_ra_hit",
 #endif
 #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
+	/* Deprecated counters. Count them in NR_VM_DEPRECATED_ITEMS */
+	"nr_unstable",
 };
 #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA || CONFIG_MEMCG */
 
+#define NR_VM_DEPRECATED_ITEMS 1
+
 #if (defined(CONFIG_DEBUG_FS) && defined(CONFIG_COMPACTION)) || \
      defined(CONFIG_PROC_FS)
 static void *frag_start(struct seq_file *m, loff_t *pos)
@@ -1661,7 +1664,8 @@ static const struct seq_operations zoneinfo_op = {
 			 NR_VM_NODE_STAT_ITEMS + \
 			 NR_VM_WRITEBACK_STAT_ITEMS + \
 			 (IS_ENABLED(CONFIG_VM_EVENT_COUNTERS) ? \
-			  NR_VM_EVENT_ITEMS : 0))
+			  NR_VM_EVENT_ITEMS : 0) + \
+			  NR_VM_DEPRECATED_ITEMS)
 
 static void *vmstat_start(struct seq_file *m, loff_t *pos)
 {
@@ -1698,7 +1702,11 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
 	all_vm_events(v);
 	v[PGPGIN] /= 2;		/* sectors -> kbytes */
 	v[PGPGOUT] /= 2;
+	v += NR_VM_EVENT_ITEMS;
 #endif
+	for (i = 0; i < NR_VM_DEPRECATED_ITEMS)
+		v[i] = 0;
+
 	return (unsigned long *)m->private + *pos;
 }
 
-- 
Michal Hocko
SUSE Labs