linux-kernel - Re: [patch 12/21] Xen-paravirt: Allocate and free vmalloc areas

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Date:	Thu, 15 Feb 2007 23:24:47 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <ak@....de>, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [patch 12/21] Xen-paravirt: Allocate and free vmalloc areas

On Thu, 15 Feb 2007 23:08:02 -0800 Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Andrew Morton wrote:
> > This won't work when CONFIG_PREEMPT=y.  The pagefault handler will see
> > in_atomic() and will scram.
> >   
> 
> Is there some other way to get the pagetable populated for the address
> range?
> 

If you really need to run atomically, that gets ugly.  Even of one were to
run handle_mm_fault() by hand, it still needs to allocate memory.

Two ugly options might be:

a) touch all the pages, then go atomic, then touch them all again.  If
   one of them faults (ie: you raced with swapout) then go back and try
   again.  Obviously susceptible to livelocking.

b) Do get_user_pages() against all the pages, then go atomic, then do
   put_page() against them all.  Of course, they can immediately get
   swapped out.

But that function's already racy against swapout and I guess it works OK. 
I don't have clue what it is actually trying to do, so I'm guessing madly
here.
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