Message-Id: <4ACC62DC0200007800018690@vpn.id2.novell.com>
Date: Wed, 07 Oct 2009 08:43:56 +0100
From: "Jan Beulich" <JBeulich@novell.com>
To: "Hugh Dickins" <hugh.dickins@tiscali.co.uk>
Cc: <linux-mm@kvack.org>, "Andrew Morton" <akpm@linux-foundation.org>,
       <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] adjust gfp mask passed on nested vmalloc()
	 invocation
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>>> Hugh Dickins <hugh.dickins@tiscali.co.uk> 06.10.09 23:58 >>>
>On Mon, 5 Oct 2009, Jan Beulich wrote:
>
>> - fix a latent bug resulting from blindly or-ing in __GFP_ZERO, since
>>   the combination of this and __GFP_HIGHMEM (possibly passed into the
>>   function) is forbidden in interrupt context
>> - avoid wasting more precious resources (DMA or DMA32 pools), when
>>   being called through vmalloc_32{,_user}()
>> - explicitly allow using high memory here even if the outer allocation
>>   request doesn't allow it, unless is collides with __GFP_ZERO
>> 
>> Signed-off-by: Jan Beulich <jbeulich@novell.com>
>
>I thought vmalloc.c was a BUG_ON(in_interrupt()) zone?
>The locking is all spin_lock stuff, not spin_lock_irq stuff.
>That's probably why your "bug" has remained "latent".

Then you probably mean BUG_ON(irqs_disabled()), which would seem
correct. But if the gfp mask massaging was needed for calling kmalloc(),
it would seem odd that the same shouldn't be needed for calling
vmalloc() recursively...

>Using HIGHMEM for internal arrays looks reasonable to me; but if
>__GFP_ZERO were a problem, wouldn't it be much cleaner to skip the
>"unless it collides" and #ifdef CONFIG_HIGHMEM !in_interrupt() stuff,
>just memset the array returned from __vmalloc_node()?

The main goal was to change the existing code as little as possible - I
did consider this alternative, but wasn't sure that would be accepted.
If you view this as the better alternative, I'll certainly modify the
patch to do it that way.

Jan

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