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

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Message-ID: <20100118133512.GC721@localhost>
Date:	Mon, 18 Jan 2010 21:35:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	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>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 5/8] vmalloc: simplify vread()/vwrite()

On Thu, Jan 14, 2010 at 05:45:26AM -0700, Nick Piggin wrote:
> On Wed, Jan 13, 2010 at 09:53:10PM +0800, Wu Fengguang wrote:
> > vread()/vwrite() is only called from kcore/kmem to access one page at a time.
> > So the logic can be vastly simplified.
> > 
> > The changes are:
> > - remove the vmlist walk and rely solely on vmalloc_to_page()
> > - replace the VM_IOREMAP check with (page && page_is_ram(pfn))
> > - rename to vread_page()/vwrite_page()
> > 
> > The page_is_ram() check is necessary because kmap_atomic() is not
> > designed to work with non-RAM pages.
> 
> I don't know if you can really do this. Previously vmlist_lock would be
> taken, which will prevent these vm areas from being freed.
>  
> > Note that even for a RAM page, we don't own the page, and cannot assume
> > it's a _PAGE_CACHE_WB page.
> 
> So why is this not a problem for your patch? I don't see how you handle
> it.

Sorry I didn't handle it. Just hope to catch attentions from someone
(ie. you :).

It's not a problem for x86_64 at all. For others I wonder if any
driver will vmalloc HIGHMEM pages with !_PAGE_CACHE_WB attribute..

So I noted the possible problem and leave it alone.

> What's the problem with the current code, exactly? I would prefer that

- unnecessary complexity to handle multi-page case, since it's always
  called to access one single page;

- the kmap_atomic() cache consistency problem, which I expressed some
  concern (without further action)

> you continue using the same vmlist locking and checking for validating
> addresses.

It's a reasonable suggestion. Kame, would you agree on killing the
kmap_atomic() and revert to the vmlist walk?

Thanks,
Fengguang
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