linux-kernel - [RFC PATCH v1 03/57] mm/memcontrol: Fix seq_buf size to save memory when PAGE

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Message-ID: <20241014105912.3207374-3-ryan.roberts@arm.com>
Date: Mon, 14 Oct 2024 11:58:10 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	David Hildenbrand <david@...hat.com>,
	Greg Marsden <greg.marsden@...cle.com>,
	Ivan Ivanov <ivan.ivanov@...e.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Kalesh Singh <kaleshsingh@...gle.com>,
	Marc Zyngier <maz@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Matthias Brugger <mbrugger@...e.com>,
	Michal Hocko <mhocko@...nel.org>,
	Miroslav Benes <mbenes@...e.cz>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Will Deacon <will@...nel.org>
Cc: Ryan Roberts <ryan.roberts@....com>,
	cgroups@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [RFC PATCH v1 03/57] 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 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@....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