linux-kernel - Re: [PATCH v3] mm: per-thread vma caching

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Date:	Wed, 26 Feb 2014 22:47:10 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>, aswin@...com,
	scott.norton@...com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: per-thread vma caching

Agree with Linus; this is starting to look pretty good.

I still have nits though :)

On Wed, Feb 26, 2014 at 4:07 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> @@ -0,0 +1,45 @@
> +#ifndef __LINUX_VMACACHE_H
> +#define __LINUX_VMACACHE_H
> +
> +#include <linux/mm.h>
> +
> +#ifdef CONFIG_MMU
> +#define VMACACHE_BITS 2
> +#else
> +#define VMACACHE_BITS 0
> +#endif

I wouldn't even both with the #ifdef here - why not just always use 2 bits ?

> +#define vmacache_flush(tsk)                                     \
> +       do {                                                     \
> +               memset(tsk->vmacache, 0, sizeof(tsk->vmacache)); \
> +       } while (0)

I think inline functions are preferred

> diff --git a/mm/nommu.c b/mm/nommu.c
> index 8740213..9a5347b 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -768,16 +768,19 @@ static void add_vma_to_mm(struct mm_struct *mm, struct vm_area_struct *vma)
>   */
>  static void delete_vma_from_mm(struct vm_area_struct *vma)
>  {
> +       int i;
>         struct address_space *mapping;
>         struct mm_struct *mm = vma->vm_mm;
> +       struct task_struct *curr = current;
>
>         kenter("%p", vma);
>
>         protect_vma(vma, 0);
>
>         mm->map_count--;
> -       if (mm->mmap_cache == vma)
> -               mm->mmap_cache = NULL;
> +       for (i = 0; i < VMACACHE_SIZE; i++)
> +               if (curr->vmacache[i] == vma)
> +                       curr->vmacache[i] = NULL;

Why is the invalidation done differently here ? shouldn't it be done
by bumping the mm's sequence number so that invalidation works accross
all threads sharing that mm ?

> +#ifndef CONFIG_MMU
> +struct vm_area_struct *vmacache_find_exact(struct mm_struct *mm,
> +                                          unsigned long start,
> +                                          unsigned long end)
> +{
> +       int i;
> +
> +       if (!vmacache_valid(mm))
> +               return NULL;
> +
> +       for (i = 0; i < VMACACHE_SIZE; i++) {
> +               struct vm_area_struct *vma = current->vmacache[i];
> +
> +               if (vma && vma->vm_start == start && vma->vm_end == end)
> +                       return vma;
> +       }
> +
> +       return NULL;
> +
> +}
> +#endif

I think the caller could do instead
vma = vmacache_find(mm, start)
if (vma && vma->vm_start == start && vma->vm_end == end) {
}

I.e. better deal with it at the call site than add a new vmacache
function for it.

These are nits, the code looks good already.

I would like to propose an LRU eviction scheme to replace your
VMACACHE_HASH mechanism; I will probably do that as a follow-up once
you have the code in andrew's tree.

Reviewed-by: Michel Lespinasse <walken@...gle.com>

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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