linux-kernel - Re: CMA: test_pages_isolated failures in alloc_contig

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Message-ID: <54584A48.9000409@hurleysoftware.com>
Date:	Mon, 03 Nov 2014 22:38:48 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Michal Nazarewicz <mina86@...a86.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	linux-mm@...ck.org
CC:	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: CMA: test_pages_isolated failures in alloc_contig_range

On 10/28/2014 12:57 PM, Michal Nazarewicz wrote:
>> On 10/28/2014 08:38 AM, Michal Nazarewicz wrote:
>>> Like Laura wrote, the message is not (should not be) a problem in
>>> itself:
>>
>> [...]
>>
>>> So as you can see cma_alloc will try another part of the cma region if
>>> test_pages_isolated fails.
>>>
>>> Obviously, if CMA region is fragmented or there's enough space for only
>>> one allocation of required size isolation failures will cause allocation
>>> failures, so it's best to avoid them, but they are not always avoidable.
>>>
>>> To debug you would probably want to add more debug information about the
>>> page (i.e. data from struct page) that failed isolation after the
>>> pr_warn in alloc_contig_range.
> 
> On Tue, Oct 28 2014, Peter Hurley <peter@...leysoftware.com> wrote:
>> If the message does not indicate an actual problem, then its printk level is
>> too high. These messages have been reported when using 3.16+ distro kernels.
> 
> I think it could be argued both ways.  The condition is not an error,
> since in many cases cma_alloc will be able to continue, but it *is* an
> undesired state.  As such it's not an error but feels to me a bit more
> then just information, hence a warning.  I don't care either way, though.

This "undesired state" is trivially reproducible on 3.16.y on the x86 arch;
a smattering of these will show up just building a distro kernel.

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