linux-kernel - [PATCH v1] mm: optimize CONFIG_PER_VMA_LOCK member placement in vm_area

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Date: Wed, 27 Mar 2024 15:35:48 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Suren Baghdasaryan <surenb@...gle.com>
Subject: [PATCH v1] mm: optimize CONFIG_PER_VMA_LOCK member placement in vm_area_struct

Currently, we end up wasting some memory in each vm_area_struct. Pahole
states that:
	[...]
	int                        vm_lock_seq;          /*    40     4 */

	/* XXX 4 bytes hole, try to pack */

	struct vma_lock *          vm_lock;              /*    48     8 */
	bool                       detached;             /*    56     1 */

	/* XXX 7 bytes hole, try to pack */
	[...]

Let's reduce the holes and memory wastage by moving the bool:
	[...]
	bool                       detached;             /*    40     1 */

	/* XXX 3 bytes hole, try to pack */

	int                        vm_lock_seq;          /*    44     4 */
	struct vma_lock *          vm_lock;              /*    48     8 */
	[...]

Effectively shrinking the vm_area_struct with CONFIG_PER_VMA_LOCK by
8 byte.

Likely, we could place "detached" in the lowest bit of vm_lock, but at
least on 64bit that won't really make a difference, so keep it simple.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 include/linux/mm_types.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 4ae4684d1add..f56739dece7a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -671,6 +671,9 @@ struct vm_area_struct {
 	};
 
 #ifdef CONFIG_PER_VMA_LOCK
+	/* Flag to indicate areas detached from the mm->mm_mt tree */
+	bool detached;
+
 	/*
 	 * Can only be written (using WRITE_ONCE()) while holding both:
 	 *  - mmap_lock (in write mode)
@@ -687,9 +690,6 @@ struct vm_area_struct {
 	 */
 	int vm_lock_seq;
 	struct vma_lock *vm_lock;
-
-	/* Flag to indicate areas detached from the mm->mm_mt tree */
-	bool detached;
 #endif
 
 	/*
-- 
2.43.2