From 3b612c8f069fecb84af277c8b90e14832749ceca Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Wed, 27 Mar 2024 15:35:48 +0100 Subject: [PATCH] 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: David Hildenbrand Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- 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 6fef56938a17..fee6561feb7e 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 /*