linux-kernel - Re: [PATCH 2/3] x86, swiotlb: Try coherent allocations with __GFP

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Message-ID: <20141202184648.GB32622@laptop.dumpdata.com>
Date:	Tue, 2 Dec 2014 13:46:48 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Joerg Roedel <jroedel@...e.de>
Cc:	Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86, swiotlb: Try coherent allocations with
 __GFP_NOWARN

On Tue, Dec 02, 2014 at 03:45:51PM +0100, Joerg Roedel wrote:
> On Mon, Dec 01, 2014 at 03:28:54PM -0500, Konrad Rzeszutek Wilk wrote:
> > On Fri, Nov 28, 2014 at 12:29:08PM +0100, Joerg Roedel wrote:
> > > diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> > > index 77dd0ad..79b2291 100644
> > > --- a/arch/x86/kernel/pci-swiotlb.c
> > > +++ b/arch/x86/kernel/pci-swiotlb.c
> > > @@ -20,6 +20,14 @@ void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> > >  {
> > >  	void *vaddr;
> > >  
> > > +	/*
> > > +	 * When booting a kdump kernel in high memory these allocations are very
> > > +	 * likely to fail, as there are by default only 8MB of low memory to
> > > +	 * allocate from. So disable the warnings from the allocator when this
> > > +	 * happens.  SWIOTLB also implements fall-backs for failed allocations.
> > > +	 */
> > > +	flags |= __GFP_NOWARN;
> > 
> > Should this perhaps then have 'if (kdump_kernel)' around it since
> > the use-case seems to be kdump related?
> 
> Hmm, I don't think this is entirely kdump specific. It can also be
> triggered on a non-kdump kernel, it is just much more unlikely. But
> maybe I should change the comment to something like:
> 
> 	/*
> 	 * Don't print a warning when the first allocation attempt
> 	 * fails. The swiotlb_alloc_coherent() function will print a
> 	 * warning when the allocation of DMA memory ultimatly failed.
> 	 */

Much better. Thank you.
> 
> This takes the kdump-specifics out of this change (in the end
> kdump-kernel loaded high is just a case where this failure is much more
> likely).

<nods>
> 
> 
> 	Joerg
> 
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