mm/memcontrol: fix seq_buf size to save memory when PAGE_SIZE is large

Previously the seq_buf used for accumulating the memory.stat output was
sized at PAGE_SIZE.  But the amount of output is invariant to PAGE_SIZE;
If 4K is enough on a 4K page system, then it should also be enough on a
64K page system, so we can save 60K on the static buffer used in
mem_cgroup_print_oom_meminfo().  Let's make it so.

This also has the beneficial side effect of removing a place in the code
that assumed PAGE_SIZE is a compile-time constant.  So this helps our
quest towards supporting boot-time page size selection.

Link: https://lkml.kernel.org/r/20241021130027.3615969-1-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -118,6 +118,7 @@ struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr)
 	return container_of(vmpr, struct mem_cgroup, vmpressure);
 }
 
+#define SEQ_BUF_SIZE SZ_4K
 #define CURRENT_OBJCG_UPDATE_BIT 0
 #define CURRENT_OBJCG_UPDATE_FLAG (1UL << CURRENT_OBJCG_UPDATE_BIT)
 
@@ -1527,7 +1528,7 @@ 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[SEQ_BUF_SIZE];
 	struct seq_buf s;
 
 	lockdep_assert_held(&oom_lock);
@@ -1553,7 +1554,7 @@ 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(":");
-	seq_buf_init(&s, buf, sizeof(buf));
+	seq_buf_init(&s, buf, SEQ_BUF_SIZE);
 	memory_stat_format(memcg, &s);
 	seq_buf_do_printk(&s, KERN_INFO);
 }
@@ -4196,12 +4197,12 @@ static int memory_events_local_show(struct seq_file *m, void *v)
 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);
+	char *buf = kmalloc(SEQ_BUF_SIZE, GFP_KERNEL);
 	struct seq_buf s;
 
 	if (!buf)
 		return -ENOMEM;
-	seq_buf_init(&s, buf, PAGE_SIZE);
+	seq_buf_init(&s, buf, SEQ_BUF_SIZE);
 	memory_stat_format(memcg, &s);
 	seq_puts(m, buf);
 	kfree(buf);