On Wed 26-04-23 13:39:18, Yosry Ahmed wrote:
> Currently, we format all the memcg stats into a buffer in
> mem_cgroup_print_oom_meminfo() and use pr_info() to dump it to the logs.
> However, this buffer is large in size. Although it is currently working
> as intended, ther is a dependency between the memcg stats buffer and the
> printk record size limit.
>
> If we add more stats in the future and the buffer becomes larger than
> the printk record size limit, or if the prink record size limit is
> reduced, the logs may be truncated.
>
> It is safer to use seq_buf_do_printk(), which will automatically break
> up the buffer at line breaks and issue small printk() calls.
>
> Refactor the code to move the seq_buf from memory_stat_format() to its
> callers, and use seq_buf_do_printk() to print the seq_buf in
> mem_cgroup_print_oom_meminfo().
>
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
No objection from me but is it possible that more printk calls (one per
line with this change correct?) would add a contention on the printk
path?
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/memcontrol.c | 27 ++++++++++++++-------------
> 1 file changed, 14 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5abffe6f8389..5922940f92c9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1551,13 +1551,10 @@ static inline unsigned long memcg_page_state_output(struct mem_cgroup *memcg,
> return memcg_page_state(memcg, item) * memcg_page_state_unit(item);
> }
>
> -static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
> +static void memory_stat_format(struct mem_cgroup *memcg, struct seq_buf *s)
> {
> - struct seq_buf s;
> int i;
>
> - seq_buf_init(&s, buf, bufsize);
> -
> /*
> * Provide statistics on the state of the memory subsystem as
> * well as cumulative event counters that show past behavior.
> @@ -1574,21 +1571,21 @@ static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
> u64 size;
>
> size = memcg_page_state_output(memcg, memory_stats[i].idx);
> - seq_buf_printf(&s, "%s %llu\n", memory_stats[i].name, size);
> + seq_buf_printf(s, "%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);
> + seq_buf_printf(s, "slab %llu\n", size);
> }
> }
>
> /* Accumulated memory events */
> - seq_buf_printf(&s, "pgscan %lu\n",
> + seq_buf_printf(s, "pgscan %lu\n",
> memcg_events(memcg, PGSCAN_KSWAPD) +
> memcg_events(memcg, PGSCAN_DIRECT) +
> memcg_events(memcg, PGSCAN_KHUGEPAGED));
> - seq_buf_printf(&s, "pgsteal %lu\n",
> + seq_buf_printf(s, "pgsteal %lu\n",
> memcg_events(memcg, PGSTEAL_KSWAPD) +
> memcg_events(memcg, PGSTEAL_DIRECT) +
> memcg_events(memcg, PGSTEAL_KHUGEPAGED));
> @@ -1598,13 +1595,13 @@ static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
> memcg_vm_event_stat[i] == PGPGOUT)
> continue;
>
> - seq_buf_printf(&s, "%s %lu\n",
> + seq_buf_printf(s, "%s %lu\n",
> vm_event_name(memcg_vm_event_stat[i]),
> memcg_events(memcg, memcg_vm_event_stat[i]));
> }
>
> /* The above should easily fit into one page */
> - WARN_ON_ONCE(seq_buf_has_overflowed(&s));
> + WARN_ON_ONCE(seq_buf_has_overflowed(s));
> }
>
> #define K(x) ((x) << (PAGE_SHIFT-10))
> @@ -1642,6 +1639,7 @@ 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];
> + struct seq_buf s;
>
> lockdep_assert_held(&oom_lock);
>
> @@ -1664,8 +1662,9 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
> pr_info("Memory cgroup stats for ");
> pr_cont_cgroup_path(memcg->css.cgroup);
> pr_cont(":");
> - memory_stat_format(memcg, buf, sizeof(buf));
> - pr_info("%s", buf);
> + seq_buf_init(&s, buf, sizeof(buf));
> + memory_stat_format(memcg, &s);
> + seq_buf_do_printk(&s, KERN_INFO);
> }
>
> /*
> @@ -6573,10 +6572,12 @@ static int memory_stat_show(struct seq_file *m, void *v)
> {
> struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
> char *buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
> + struct seq_buf s;
>
> if (!buf)
> return -ENOMEM;
> - memory_stat_format(memcg, buf, PAGE_SIZE);
> + seq_buf_init(&s, buf, PAGE_SIZE);
> + memory_stat_format(memcg, &s);
> seq_puts(m, buf);
> kfree(buf);
> return 0;
> --
> 2.40.1.495.gc816e09b53d-goog
--
Michal Hocko
SUSE Labs
