memcg: use seq_buf_do_printk() with mem_cgroup_print_oom_meminfo()

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().

Link: https://lkml.kernel.org/r/20230428132406.2540811-2-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -1580,13 +1580,10 @@ 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.
@@ -1603,21 +1600,21 @@ 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));
@@ -1627,13 +1624,13 @@ 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))
@@ -1671,6 +1668,7 @@ 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);
 
@@ -1693,8 +1691,9 @@ 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);
 }
 
 /*
@@ -6635,10 +6634,12 @@ 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;