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Date: Tue, 26 Apr 2011 20:53:30 +1000
From: NeilBrown <neilb@suse.de>
To: Mel Gorman <mgorman@suse.de>
Cc: Linux-MM <linux-mm@kvack.org>, Linux-Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 03/13] mm: Introduce __GFP_MEMALLOC to allow access to
 emergency reserves
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On Tue, 26 Apr 2011 11:36:46 +0100 Mel Gorman <mgorman@suse.de> wrote:

> On Tue, Apr 26, 2011 at 07:49:47PM +1000, NeilBrown wrote:

> > Maybe a
> >    WARN_ON((gfp_mask & __GFP_MEMALLOC) && (gfp_mask & __GFP_NOMEMALLOC));
> > might be wise?
> > 
> 
> Both MEMALLOC and NOMEMALLOC are related to PFMEMALLOC reserves so
> it's reasonable for them to have similar names. This warning will
> also trigger because it's a combination of flags that does happen.
> 
> Consider for example
> 
> any interrupt
>   -> __netdev_alloc_skb		(mask == GFP_ATOMIC)
>     -> __alloc_skb		(mask == GFP_ATOMIC)
>        if (sk_memalloc_socks() && (flags & SKB_ALLOC_RX))
>                gfp_mask |= __GFP_MEMALLOC;
> 				(mask == GFP_ATOMIC|__GFP_NOMEMALLOC)
>       -> __kmalloc_reserve
> 		First attempt tries to avoid reserves so adds __GFP_MEMALLOC
> 				(mask == GFP_ATOMIC|__GFP_NOMEMALLOC|__GFP_MEMALLOC)
> 

You have the "NO"s mixed up a bit which confused me for a while :-)
But I see your point - I guess the WARN_ON isn't really needed.


> You're right in that __GFP_NOMEMALLOC overrides __GFP_MEMALLOC so that
> could do with a note.
> 

Thanks,

NeilBrown