[RFC PATCH v1 03/57] mm/memcontrol: Fix seq_buf size to save memory when PAGE_SIZE is large

Ryan Roberts
2024-10-14T11:27:54Z

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 om 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.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

mm/memcontrol.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d563fb515766b..c5f9195f76c65 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -95,6 +95,7 @@ static DECLARE_WAIT_QUEUE_HEAD(memcg_cgwb_frn_waitq);

#define THRESHOLDS_EVENTS_TARGET 128
#define SOFTLIMIT_EVENTS_TARGET 1024
+#define SEQ_BUF_SIZE SZ_4K

static inline bool task_is_dying(void)
{
@@ -1519,7 +1520,7 @@ 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);
@@ -1545,7 +1546,7 @@ 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);
}
@@ -4158,12 +4159,12 @@ 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);
--
2.43.0