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

[Michal Hocko]

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!

>  
-- 
Michal Hocko
SUSE Labs