Commit 3b612c8f authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

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.

Link: https://lkml.kernel.org/r/20240327143548.744070-1-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 07db63a2

include/linux/mm_types.h

+3 −3
Original line number Diff line number Diff line
@@ -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



	/*