Re: [PATCH v5 22/32] mm/memcontrol.c: Convert to printbuf

From: Michal Hocko
Date: Mon Aug 08 2022 - 08:48:10 EST


On Mon 08-08-22 11:48:53, Michal Hocko wrote:
> On Mon 08-08-22 03:41:18, Matthew Wilcox wrote:
> > From: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> >
> > This converts memory_stat_format() from seq_buf to printbuf. Printbuf is
> > simalar to seq_buf except that it heap allocates the string buffer:
> > here, we were already heap allocating the buffer with kmalloc() so the
> > conversion is trivial.
>
> The changelog probably needs an update because the oom path doesn't
> allocated and for somebody just reading this patch in isolation the
> PRINTBUF_EXTERN doesn't really seem very obvious.
>
> > Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
>
> > -static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
> > +static void memory_stat_format(struct printbuf *out, struct mem_cgroup *memcg)
> > {
> > - struct seq_buf s;
> > int i;
> >
> > - seq_buf_init(&s, buf, bufsize);
> > -
>
> When is the buffer cleared?
>
> > /*
> > * Provide statistics on the state of the memory subsystem as
> > * well as cumulative event counters that show past behavior.
> [...]
> > #define K(x) ((x) << (PAGE_SHIFT-10))
> > @@ -1573,7 +1570,8 @@ void mem_cgroup_print_oom_context(struct mem_cgroup *memcg, struct task_struct *
> > void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
> > {
> > /* Use static buffer, for the caller is holding oom_lock. */
> > - static char buf[PAGE_SIZE];
> > + static char _buf[PAGE_SIZE];
> > + struct printbuf buf = PRINTBUF_EXTERN(_buf, sizeof(_buf));
> >
> > lockdep_assert_held(&oom_lock);
>
> the buffer is static here!

And answering to myself. THere is an internal pos field which gets
initialized by PRINTBUF_EXTERN even when not mentioned explicitly
because it gets a default 0 initialization

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
--
Michal Hocko
SUSE Labs