linux-kernel - Re: [PATCH v2] mm/hugetlb: fix total hugetlbfs pages count when memory overcommit accouting

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Date:	Thu, 14 Mar 2013 13:58:27 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Hillf Danton <dhillf@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/hugetlb: fix total hugetlbfs pages count when
 memory overcommit accouting

On Thu 14-03-13 19:24:11, Wanpeng Li wrote:
> On Thu, Mar 14, 2013 at 12:09:27PM +0100, Michal Hocko wrote:
> >On Thu 14-03-13 18:49:49, Wanpeng Li wrote:
> >> Changelog:
> >>  v1 -> v2:
> >>   * update patch description, spotted by Michal
> >> 
> >> hugetlb_total_pages() does not account for all the supported hugepage
> >> sizes.
> >
> >> This can lead to incorrect calculation of the total number of
> >> page frames used by hugetlb. This patch corrects the issue.
> >
> 
> Hi Michal,
> 
> >Sorry to be so picky but this doesn't tell us much. Why do we need to
> >have the total number of hugetlb pages?
> >
> >What about the following:
> >"hugetlb_total_pages is used for overcommit calculations but the
> >current implementation considers only default hugetlb page size (which
> >is either the first defined hugepage size or the one specified by
> >default_hugepagesz kernel boot parameter).
> >
> >If the system is configured for more than one hugepage size (which is
> >possible since a137e1cc hugetlbfs: per mount huge page sizes) then
> >the overcommit estimation done by __vm_enough_memory (resp. shown by
> >meminfo_proc_show) is not precise - there is an impression of more
> >available/allowed memory. This can lead to an unexpected ENOMEM/EFAULT
> >resp. SIGSEGV when memory is accounted."
> >
> 
> Fair enough, thanks. :-)
> 
> >I think this is also worth pushing to the stable tree (it goes back to
> >2.6.27)
> >
> 
> Yup, I will Cc Greg in next version. 

Ccing Greg doesn't help. All that is required is:
Cc: stable@...r.kernel.org # 2.6.27+

> >> Testcase:
> >> boot: hugepagesz=1G hugepages=1
> >> before patch:
> >> egrep 'CommitLimit' /proc/meminfo
> >> CommitLimit:     55434168 kB
> >> after patch:
> >> egrep 'CommitLimit' /proc/meminfo
> >> CommitLimit:     54909880 kB
> >
> >This gives some more confusion to a reader because there is only
> >something like 500M difference here without any explanation.
> >
> 
> the default overcommit ratio is 50.

And that part was missing in the description...
[...]
-- 
Michal Hocko
SUSE Labs
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