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SubjectRe: [PATCH 2/2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.
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
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