linux-kernel - Re: [PATCH 5/8] vmalloc: simplify vread()/vwrite()

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Date:	Tue, 19 Jan 2010 11:23:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 5/8] vmalloc: simplify vread()/vwrite()

On Tue, 19 Jan 2010 09:33:03 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:
> > The whole thing looks stupid though, apparently kmap is used to avoid "the
> > lock". But the lock is already held. We should just use the vmap
> > address.
> 
> Yes. I wonder why Kame introduced kmap_atomic() in d0107eb07 -- given
> that he at the same time fixed the order of removing vm_struct and
> vmap in dd32c279983b.
> 
Hmm...I must check my thinking again before answering..

vmalloc/vmap is constructed by 2 layer.
	- vmalloc layer....guarded by vmlist_lock.
	- vmap layer   ....gurderd by purge_lock. etc.

Now, let's see how vmalloc() works. It does job in 2 steps.
vmalloc():
  - allocate vmalloc area to the list under vmlist_lock.
	- map pages.
vfree()
  - free vmalloc area from the list under vmlist_lock.
	- unmap pages under purge_lock.

Now. vread(), vwrite() just take vmlist_lock, doesn't take purge_lock().
It walks page table and find pte entry, page, kmap and access it.

Oh, yes. It seems it's safe without kmap. But My concern is percpu allocator.

It uses get_vm_area() and controls mapped pages by themselves and
map/unmap pages by with their own logic. vmalloc.c is just used for
alloc/free virtual address. 

Now, vread()/vwrite() just holds vmlist_lock() and walk page table
without no guarantee that the found page is stably mapped. So, I used kmap.

If I miss something, I'm very sorry to add such kmap.

Thanks,
-Kame

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