netdev - Re: in_irq_or

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Message-ID: <20170329181226.GA8256@bombadil.infradead.org>
Date:   Wed, 29 Mar 2017 11:12:26 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tariq Toukan <tariqt@...lanox.com>, netdev@...r.kernel.org,
        akpm@...ux-foundation.org, linux-mm <linux-mm@...ck.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        linux-kernel@...r.kernel.org
Subject: Re: in_irq_or_nmi()

On Wed, Mar 29, 2017 at 11:19:49AM +0200, Peter Zijlstra wrote:
> On Wed, Mar 29, 2017 at 10:59:28AM +0200, Jesper Dangaard Brouer wrote:
> > On Wed, 29 Mar 2017 10:12:19 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > > No, that's horrible. Also, wth is this about? A memory allocator that
> > > needs in_nmi()? That sounds beyond broken.
> > 
> > It is the other way around. We want to exclude NMI and HARDIRQ from
> > using the per-cpu-pages (pcp) lists "order-0 cache" (they will
> > fall-through using the normal buddy allocator path).
> 
> Any in_nmi() code arriving at the allocator is broken. No need to fix
> the allocator.

That's demonstrably true.  You can't grab a spinlock in NMI code and
the first thing that happens if this in_irq_or_nmi() check fails is ...
        spin_lock_irqsave(&zone->lock, flags);
so this patch should just use in_irq().

(the concept of NMI code needing to allocate memory was blowing my mind
a little bit)