linux-kernel - [PATCH] mm/hugetlb: sort out global lock annotations

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Message-ID: <20240828160704.1425767-1-mjguzik@gmail.com>
Date: Wed, 28 Aug 2024 18:07:04 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: muchun.song@...ux.dev,
	akpm@...ux-foundation.org
Cc: dave@...olabs.net,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] mm/hugetlb: sort out global lock annotations

The mutex array pointer shares a cacheline with the spinlock:
ffffffff84187480 B hugetlb_fault_mutex_table
ffffffff84187488 B hugetlb_lock

This is because the former is annotated with a macro forcing cacheline
alignment. I suspect it was meant to be the variant which on top of it
makes sure the object does not share the cacheline with anyone.

Since array pointer itself is de facto read-only such an annotation does
not make sense there anyway. Instead mark it __ro_after_init along with
the size var.

Do however move the spinlock out of the way.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

I did not benchmark any of it, looks like basic sanity to me.

This came up as a side effect of an unrelated discussion.

 mm/hugetlb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4461d27f7453..1a833f016847 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -72,14 +72,14 @@ static unsigned int default_hugepages_in_node[MAX_NUMNODES] __initdata;
  * Protects updates to hugepage_freelists, hugepage_activelist, nr_huge_pages,
  * free_huge_pages, and surplus_huge_pages.
  */
-DEFINE_SPINLOCK(hugetlb_lock);
+__cacheline_aligned_in_smp DEFINE_SPINLOCK(hugetlb_lock);
 
 /*
  * Serializes faults on the same logical page.  This is used to
  * prevent spurious OOMs when the hugepage pool is fully utilized.
  */
-static int num_fault_mutexes;
-struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp;
+static __ro_after_init int num_fault_mutexes;
+__ro_after_init struct mutex *hugetlb_fault_mutex_table;
 
 /* Forward declaration */
 static int hugetlb_acct_memory(struct hstate *h, long delta);
-- 
2.43.0