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

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Date:	Thu, 21 Jan 2010 13:05:21 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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 Mon, Jan 18, 2010 at 07:23:43PM -0700, KAMEZAWA Hiroyuki wrote:
> 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.

Ah Thanks for explanation!

I did some audit and find that

- set_memory_uc(), set_memory_array_uc(), set_pages_uc(),
  set_pages_array_uc() are called EFI code and various video drivers,
  all of them don't touch HIGHMEM RAM

- Kame: ioremap() won't allow remap of physical RAM

So kmap_atomic() is safe.  Let's just settle on this patch?

Thanks,
Fengguang
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