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SubjectRe: [PATCH 2/2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.
On Tue 07-04-20 13:24:12, Michal Hocko wrote:
> On Tue 07-04-20 12:25:15, Jan Kara wrote:
> > > @@ -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)
^^ ; i++ here

but otherwise yes, this is what I meant. Thanks!

> + v[i] = 0;
> +
> return (unsigned long *)m->private + *pos;
> }

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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