linux-kernel - Re: [PATCH v5 2/2] mm: hugetlb: proc: add HugetlbPages field to /proc/PID/status

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Message-ID: <20150826063813.GA25196@dhcp22.suse.cz>
Date:	Wed, 26 Aug 2015 08:38:14 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Jörn Engel <joern@...estorage.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v5 2/2] mm: hugetlb: proc: add HugetlbPages field to
 /proc/PID/status

On Tue 25-08-15 16:23:34, David Rientjes wrote:
> On Mon, 24 Aug 2015, Michal Hocko wrote:
> 
> > The current implementation makes me worry. Is the per hstate break down
> > really needed? The implementation would be much more easier without it.
> > 
> 
> Yes, it's needed.  It provides a complete picture of what statically 
> reserved hugepages are in use and we're not going to change the 
> implementation when it is needed to differentiate between variable hugetlb 
> page sizes that risk breaking existing userspace parsers.

I thought the purpose was to give the amount of hugetlb based
resident memory. At least this is what Jörn was asking for AFAIU.
/proc/<pid>/status should be as lightweight as possible. The current
implementation is quite heavy as already pointed out. So I am really
curious whether this is _really_ needed. I haven't heard about a real
usecase except for top displaying HRss which doesn't need the break
down values. You have brought that up already
http://marc.info/?l=linux-mm&m=143941143109335&w=2 and nobody actually
asked for it. "I do not mind having it" is not an argument for inclusion
especially when the implementation is more costly and touches hot paths.

> > If you have 99% of hugetlb pages then your load is rather specific and I
> > would argue that /proc/<pid>/smaps (after patch 1) is a much better way to
> > get what you want.
> 
> Some distributions change the permissions of smaps, as already stated, for 
> pretty clear security reasons since it can be used to defeat existing 
> protection.  There's no reason why hugetlb page usage should not be 
> exported in the same manner and location as memory usage.

/proc/<pid>/status provides only per-memory-type break down information
(locked, data, stack, etc...). Different hugetlb sizes are still a
hugetlb memory. So I am not sure I understand you argument here.
-- 
Michal Hocko
SUSE Labs
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