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

Michal Hocko
2022-04-22T12:28:08Z

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