linux-kernel - Re: [PATCH v2] mm: cma: fix CMA aligned offset calculation

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Date:	Fri, 27 Feb 2015 17:18:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Danesh Petigara <dpetigara@...adcom.com>
Cc:	<m.szyprowski@...sung.com>, <mina86@...a86.com>,
	<iamjoonsoo.kim@....com>, <aneesh.kumar@...ux.vnet.ibm.com>,
	<laurent.pinchart+renesas@...asonboard.com>,
	<gregory.0xf0@...il.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH v2] mm: cma: fix CMA aligned offset calculation

On Fri, 27 Feb 2015 17:07:28 -0800 Danesh Petigara <dpetigara@...adcom.com> wrote:

> On 2/27/2015 3:54 PM, Andrew Morton wrote:
> > On Fri, 27 Feb 2015 15:52:56 -0800 Danesh Petigara <dpetigara@...adcom.com> wrote:
> > 
> >> On 2/27/2015 1:24 PM, Andrew Morton wrote:
> >>> On Tue, 24 Feb 2015 15:39:45 -0800 Danesh Petigara <dpetigara@...adcom.com> wrote:
> >>>
> >>>> The CMA aligned offset calculation is incorrect for
> >>>> non-zero order_per_bit values.
> >>>>
> >>>> For example, if cma->order_per_bit=1, cma->base_pfn=
> >>>> 0x2f800000 and align_order=12, the function returns
> >>>> a value of 0x17c00 instead of 0x400.
> >>>>
> >>>> This patch fixes the CMA aligned offset calculation.
> >>>
> >>> When fixing a bug please always describe the end-user visible effects
> >>> of that bug.
> >>>
> >>> Without that information others are unable to understand why you are
> >>> recommending a -stable backport.
> >>>
> >>
> >> Thank you for the feedback. I had no crash logs to show, nevertheless, I
> >> agree that a sentence describing potential effects of the bug would've
> >> helped.
> > 
> > What was the reason for adding a cc:stable?
> > 
> 
> It was added since the commit that introduced the incorrect logic
> (b5be83e) was already picked up by v3.19.

argh.

afaict the bug will, under some conditions cause cma_alloc() to report
that no suitable free area is available in the arena when in fact such
regions *are* available.  So it's effectively a bogus ENOMEM.

Correct?  If so, what are the conditions under which this will occur?

This isn't hard - I want to know what the patch *does*!
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