Re: [PATCH v2 3/8] mm/memcontrol.c: Convert to printbuf

[Michal Hocko <mhocko@suse.com>]

On Thu 21-04-22 19:48:32, Kent Overstreet wrote:
> 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.

What is the advantage of changing a well tested seq_buf with a different
way to do the same thing here?

I do not see this to be a noticeable simplification of the existing
code. The only advantage I can see is that the string storage allocation
is implicit and it would expand in case we ever overflow over a single
page. But is this really worth the code churn?

> Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
> ---
>  mm/memcontrol.c | 68 ++++++++++++++++++++++++-------------------------
>  1 file changed, 33 insertions(+), 35 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 36e9f38c91..4cb0b7bc1c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -61,7 +61,7 @@
>  #include <linux/file.h>
>  #include <linux/tracehook.h>
>  #include <linux/psi.h>
> -#include <linux/seq_buf.h>
> +#include <linux/printbuf.h>
>  #include "internal.h"
>  #include <net/sock.h>
>  #include <net/ip.h>
> @@ -1436,13 +1436,9 @@ static inline unsigned long memcg_page_state_output(struct mem_cgroup *memcg,
>  
>  static char *memory_stat_format(struct mem_cgroup *memcg)
>  {
> -	struct seq_buf s;
> +	struct printbuf buf = PRINTBUF;
>  	int i;
>  
> -	seq_buf_init(&s, kmalloc(PAGE_SIZE, GFP_KERNEL), PAGE_SIZE);
> -	if (!s.buffer)
> -		return NULL;
> -
>  	/*
>  	 * Provide statistics on the state of the memory subsystem as
>  	 * well as cumulative event counters that show past behavior.
> @@ -1459,49 +1455,51 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
>  		u64 size;
>  
>  		size = memcg_page_state_output(memcg, memory_stats[i].idx);
> -		seq_buf_printf(&s, "%s %llu\n", memory_stats[i].name, size);
> +		pr_buf(&buf, "%s %llu\n", memory_stats[i].name, size);
>  
>  		if (unlikely(memory_stats[i].idx == NR_SLAB_UNRECLAIMABLE_B)) {
>  			size += memcg_page_state_output(memcg,
>  							NR_SLAB_RECLAIMABLE_B);
> -			seq_buf_printf(&s, "slab %llu\n", size);
> +			pr_buf(&buf, "slab %llu\n", size);
>  		}
>  	}
>  
>  	/* Accumulated memory events */
>  
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(PGFAULT),
> -		       memcg_events(memcg, PGFAULT));
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(PGMAJFAULT),
> -		       memcg_events(memcg, PGMAJFAULT));
> -	seq_buf_printf(&s, "%s %lu\n",  vm_event_name(PGREFILL),
> -		       memcg_events(memcg, PGREFILL));
> -	seq_buf_printf(&s, "pgscan %lu\n",
> -		       memcg_events(memcg, PGSCAN_KSWAPD) +
> -		       memcg_events(memcg, PGSCAN_DIRECT));
> -	seq_buf_printf(&s, "pgsteal %lu\n",
> -		       memcg_events(memcg, PGSTEAL_KSWAPD) +
> -		       memcg_events(memcg, PGSTEAL_DIRECT));
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(PGACTIVATE),
> -		       memcg_events(memcg, PGACTIVATE));
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(PGDEACTIVATE),
> -		       memcg_events(memcg, PGDEACTIVATE));
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(PGLAZYFREE),
> -		       memcg_events(memcg, PGLAZYFREE));
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(PGLAZYFREED),
> -		       memcg_events(memcg, PGLAZYFREED));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(PGFAULT),
> +	       memcg_events(memcg, PGFAULT));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(PGMAJFAULT),
> +	       memcg_events(memcg, PGMAJFAULT));
> +	pr_buf(&buf, "%s %lu\n",  vm_event_name(PGREFILL),
> +	       memcg_events(memcg, PGREFILL));
> +	pr_buf(&buf, "pgscan %lu\n",
> +	       memcg_events(memcg, PGSCAN_KSWAPD) +
> +	       memcg_events(memcg, PGSCAN_DIRECT));
> +	pr_buf(&buf, "pgsteal %lu\n",
> +	       memcg_events(memcg, PGSTEAL_KSWAPD) +
> +	       memcg_events(memcg, PGSTEAL_DIRECT));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(PGACTIVATE),
> +	       memcg_events(memcg, PGACTIVATE));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(PGDEACTIVATE),
> +	       memcg_events(memcg, PGDEACTIVATE));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(PGLAZYFREE),
> +	       memcg_events(memcg, PGLAZYFREE));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(PGLAZYFREED),
> +	       memcg_events(memcg, PGLAZYFREED));
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(THP_FAULT_ALLOC),
> -		       memcg_events(memcg, THP_FAULT_ALLOC));
> -	seq_buf_printf(&s, "%s %lu\n", vm_event_name(THP_COLLAPSE_ALLOC),
> -		       memcg_events(memcg, THP_COLLAPSE_ALLOC));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(THP_FAULT_ALLOC),
> +	       memcg_events(memcg, THP_FAULT_ALLOC));
> +	pr_buf(&buf, "%s %lu\n", vm_event_name(THP_COLLAPSE_ALLOC),
> +	       memcg_events(memcg, THP_COLLAPSE_ALLOC));
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> -	/* The above should easily fit into one page */
> -	WARN_ON_ONCE(seq_buf_has_overflowed(&s));
> +	if (buf.allocation_failure) {
> +		printbuf_exit(&buf);
> +		return NULL;
> +	}
>  
> -	return s.buffer;
> +	return buf.buf;
>  }
>  
>  #define K(x) ((x) << (PAGE_SHIFT-10))
> -- 
> 2.35.2

-- 
Michal Hocko
SUSE Labs