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From: Suren Baghdasaryan <surenb@google.com>
Date: Wed, 27 Mar 2024 12:58:36 -0700
Message-ID: <CAJuCfpF4EPeGN9EcPBb608LEv7=7d31O+-33JDtQEsoGaYgWkg@mail.gmail.com>
Subject: Re: [PATCH v1] mm: optimize CONFIG_PER_VMA_LOCK member placement in vm_area_struct
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, 
	Andrew Morton <akpm@linux-foundation.org>
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On Wed, Mar 27, 2024 at 7:36=E2=80=AFAM David Hildenbrand <david@redhat.com=
> wrote:
>
> 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@linux-foundation.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Looks reasonable to me. Thanks!

> ---
>  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
>