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

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Date:   Mon, 8 Aug 2022 14:48:03 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, pmladek@...e.com,
        Kent Overstreet <kent.overstreet@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH v5 22/32] mm/memcontrol.c: Convert to printbuf

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@...il.com>
> > 
> > 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@...il.com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> 
> > -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@...e.com>
-- 
Michal Hocko
SUSE Labs