linux-kernel - Re: [PATCH v3] mm: simplify find_vma

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Date:	Tue, 13 Dec 2011 13:25:41 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Andrew Morton (commit_signer:15/23=65%)" <akpm@...ux-foundation.org>,
	"Hugh Dickins (commit_signer:7/23=30%)" <hughd@...gle.com>,
	"Peter Zijlstra (commit_signer:4/23=17%)" <a.p.zijlstra@...llo.nl>,
	"Shaohua Li (commit_signer:3/23=13%)" <shaohua.li@...el.com>
Subject: Re: [PATCH v3] mm: simplify find_vma_prev

On Mon, 12 Dec 2011 10:31:57 -0500
KOSAKI Motohiro <kosaki.motohiro@...il.com> wrote:

> (12/12/11 4:27 AM), KAMEZAWA Hiroyuki wrote:
> > On Mon, 12 Dec 2011 09:49:30 +0900
> > KAMEZAWA Hiroyuki<kamezawa.hiroyu@...fujitsu.com>  wrote:
> >
> >> On Fri,  9 Dec 2011 17:48:40 -0500
> >> kosaki.motohiro@...il.com wrote:
> >>
> >>> From: KOSAKI Motohiro<kosaki.motohiro@...fujitsu.com>
> >>>
> >>> 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@...fujitsu.com>
> >>
> >> Reviewed-by: KAMEZAWA Hiroyuki<kamezawa.hiroyu@...fujitsu.com>
> >>
> >
> > 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@...fujitsu.com>
> > 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?

yes.

> If so, it is most tricky part of this patch, I strongly hope you write
> a comment more.
> 

Sure.
-Kame




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