Re: [PATCH v3] mm: simplify find_vma_prev

[KOSAKI Motohiro]

(12/12/11 4:27 AM), KAMEZAWA Hiroyuki wrote:
> On Mon, 12 Dec 2011 09:49:30 +0900
> KAMEZAWA Hiroyuki<kamezawa.hiroyu@xxxxxxxxxxxxxx>  wrote:
>
> > On Fri,  9 Dec 2011 17:48:40 -0500
> > kosaki.motohiro@xxxxxxxxx wrote:
> >
> > > From: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>
> > >
> > > commit 297c5eee37 (mm: make the vma list be doubly linked) added
> > > vm_prev member into vm_area_struct. Therefore we can simplify
> > > find_vma_prev() by using it. Also, this change help to improve
> > > page fault performance because it has strong locality of reference.
> > >
> > > Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>
> >
> > Reviewed-by: KAMEZAWA Hiroyuki<kamezawa.hiroyu@xxxxxxxxxxxxxx>
>
> Hmm, your work remind me of a patch I tried in past.
> Here is a refleshed one...how do you think ?
>
> ==
>  From c0261936fc01322d06425731d33f38b2021e8067 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki<kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Date: Mon, 12 Dec 2011 18:31:19 +0900
> Subject: [PATCH] per thread vma cache.
>
> This is a toy patch. How do you think ?
>
> This is a patch for per-thread mmap_cache without heavy atomic ops.
>
> I'm sure overhead of find_vma() is pretty small in usual application
> and this will not show good improvement. But I think, if we need
> to have cache of vma, it should be per thread rather than per mm.

Agreed. per-thread is better.


> This patch adds thread->mmap_cache, a pointer for vm_area_struct
> and update it appropriately. Because we have no refcnt on vm_area_struct,
> thread->mmap_cache may be a stale pointer. This patch detects stale
> pointer by checking
>
>      - thread->mmap_cache is one of SLABs in vm_area_cachep.
>      - thread->mmap_cache->vm_mm == mm.
>
> vma->vm_mm will be cleared before kmem_cache_free() by this patch.

Do you mean the cache can make mishit with unrelated vma when freed vma 
was reused?
If so, it is most tricky part of this patch, I strongly hope you write
a comment more.

Thank you.

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