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

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Date:	Mon, 12 Dec 2011 10:31:57 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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

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

Thank you.

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