linux-kernel - Re: [PATCH] Hugetlb: stop race when reading proc meminfo

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Date:	Thu, 3 Jul 2008 09:13:17 -0500
From:	"Adam Litke" <aglitke@...il.com>
To:	"Nish Aravamudan" <nish.aravamudan@...il.com>
Cc:	"Eric Paris" <eparis@...hat.com>, wli@...omorphy.com,
	linux-kernel@...r.kernel.org, agl@...ibm.com
Subject: Re: [PATCH] Hugetlb: stop race when reading proc meminfo

I agree with Nish on both of his points.  While it is slightly
inconvenient for /proc/meminfo to occasionally display hugetlb pool
counters that, together, don't make sense, it's worth living with for
the two reasons he stated.  These counters are for informational
purposes only and relying upon them will already get you into trouble
in many other ways.  We also don't want to give any user the privilege
to mount a denial of service attack on hugetlb applications.

On Wed, Jul 2, 2008 at 5:51 PM, Nish Aravamudan
<nish.aravamudan@...il.com> wrote:
> On 6/19/08, Eric Paris <eparis@...hat.com> wrote:
>> minor nit that hugetlb_report_{,node_}meminfo() does not lock the
>>  reading of nr_huge_pages, free_huge_pages, and friends so this is not an
>>  atomic set of information put into the buffer.  If /proc/meminfo is read
>>  while the number of hugetlb pages is in flux it is possible to get
>>  incorrect output such as:
>>
>>  HugePages_Total:     7
>>  HugePages_Free:      8
>>  HugePages_Rsvd:      0
>>  Hugepagesize:     4096 kB
>>
>>  (test available at https://bugzilla.redhat.com/attachment.cgi?id=309864)
>>
>>  With the patch we beat on a number of boxes for hours with the above
>>  test and saw no inconsistencies in the meminfo output.
>>
>>  Signed-off-by: Eric Paris <eparis@...hat.com>
>
> While I understand the spirit of this patch, I'm not sure it's really
> necessary. We've never bothered locking the output in the proc file
> before because the information is inherently racy. It only gives you a
> view of what was...if any application tries to make a decision based
> upon that information (unless it has global control of the system, or
> is a testcase, arguably), then it is already racy -- that is, the
> numbers you read at one point in time have no bearing on whether those
> pages will actually be free/available, etc when you actually need
> them.
>
> That being said, I don't feel to strongly about it. Just seems like it
> might be unnecessary for an interface we know not to be fully accurate
> to begin with, but maybe consistency is worth a bit of extra locking.
>
> That actually also leads me to wonder if maybe the locking is not
> there currently to avoid letting readers of proc files from blocking
> users of hugepages?
>
> Thanks,
> Nish
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-- 
--
Adam Litke ( agl at us.ibm.com )
IBM Linux Technology Center
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