+ mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct.patch added to mm-unstable branch

+ mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: optimize CONFIG_PER_VMA_LOCK member placement in vm_area_struct
has been added to the -mm mm-unstable branch.  Its filename is
     mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm: optimize CONFIG_PER_VMA_LOCK member placement in vm_area_struct
Date: Wed, 27 Mar 2024 15:35:48 +0100

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: David Hildenbrand <david@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm_types.h |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/include/linux/mm_types.h~mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct
+++ a/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
 
 	/*
_

Patches currently in -mm which might be from david@redhat.com are

mm-secretmem-fix-gup-fast-succeeding-on-secretmem-folios.patch
mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly.patch
mm-madvise-dont-perform-madvise-vma-walk-for-madv_populate_readwrite.patch
mm-userfaultfd-dont-place-zeropages-when-zeropages-are-disallowed.patch
s390-mm-re-enable-the-shared-zeropage-for-pv-and-skeys-kvm-guests.patch
mm-convert-folio_estimated_sharers-to-folio_likely_mapped_shared.patch
mm-convert-folio_estimated_sharers-to-folio_likely_mapped_shared-fix.patch
selftests-memfd_secret-add-vmsplice-test.patch
mm-merge-folio_is_secretmem-and-folio_fast_pin_allowed-into-gup_fast_folio_allowed.patch
mm-optimize-config_per_vma_lock-member-placement-in-vm_area_struct.patch