netdev - Re: [PATCH 04/16] mm: allow PF_MEMALLOC from softirq context

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Date:	Wed, 27 Jun 2012 09:26:14 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>, Neil Brown <neilb@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Christie <michaelc@...wisc.edu>,
	Eric B Munson <emunson@...bm.net>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH 04/16] mm: allow PF_MEMALLOC from softirq context

On Tue, Jun 26, 2012 at 06:55:13PM +0200, Sebastian Andrzej Siewior wrote:
> On Fri, Jun 22, 2012 at 03:30:31PM +0100, Mel Gorman wrote:
> > This is needed to allow network softirq packet processing to make
> > use of PF_MEMALLOC.
> 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index b6c0727..5c6d9c6 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2265,7 +2265,11 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> >  	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> >  		if (gfp_mask & __GFP_MEMALLOC)
> >  			alloc_flags |= ALLOC_NO_WATERMARKS;
> > -		else if (likely(!(gfp_mask & __GFP_NOMEMALLOC)) && !in_interrupt())
> > +		else if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
> > +			alloc_flags |= ALLOC_NO_WATERMARKS;
> > +		else if (!in_interrupt() &&
> > +				((current->flags & PF_MEMALLOC) ||
> > +				 unlikely(test_thread_flag(TIF_MEMDIE))))
> >  			alloc_flags |= ALLOC_NO_WATERMARKS;
> >  	}
> 
> You allocate in RX path with __GFP_MEMALLOC and your sk->sk_allocation has
> also __GFP_MEMALLOC set. That means you should get ALLOC_NO_WATERMARKS in
> alloc_flags.

In the cases where they are annotated correctly, yes. It is recordeed if
the page gets allocated from the PFMEMALLOC reserves. If the received
packet is not SOCK_MEMALLOC and the page was allocated from PFMEMALLOC
reserves it is then discarded and the packet must be retransmitted.

> Is this to done to avoid GFP annotations in skb_share_check() and
> friends on your __netif_receive_skb() path?
> 

I don't get your question as the annotations are not being avoided. If they
are set, they are used. In the __netif_receive_skb path, PF_MEMALLOC is
set for PFMEMALLOC skbs to avoid having to annotate every single allocation
call site.

-- 
Mel Gorman
SUSE Labs
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