linux-kernel - Re: [PATCH] mm: export NR_SHMEM via sysinfo(2) / si

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Message-ID: <CAHGf_=qahsLFA4763KzFf5CgvgcE1cjxJrzNafQR49-cNJZKuw@mail.gmail.com>
Date:	Wed, 25 Jun 2014 20:12:20 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	Motohiro Kosaki <Motohiro.Kosaki@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: export NR_SHMEM via sysinfo(2) / si_meminfo() interfaces

> I agree that reporting the amount of shared pages in that historically fashion
> might not be interesting for userspace tools resorting to sysinfo(2),
> nowadays.
>
> OTOH, our documentation implies we do return shared memory there, and FWIW,
> considering the other places we do export the "shared memory" concept to
> userspace nowadays, we are suggesting it's the amount of tmpfs/shmem, and not the
> amount of shared mapped pages it historiacally represented once. What is really
> confusing is having a field that supposedely/expectedely would return the amount
> of shmem to userspace queries, but instead returns a hard-coded zero (0).
>
> I could easily find out that there were some user complaint/confusion on this
> semantic inconsistency in the past, as in:
> https://groups.google.com/forum/#!topic/comp.os.linux.development.system/ogWVn6XdvGA
>
> or in:
> http://marc.info/?l=net-snmp-cvs&m=132148788500667
>
> which suggests users seem to always have understood it as being shmem/tmpfs
> usage, as the /proc/meminfo field "MemShared" was tied direclty to
> sysinfo.sharedram. Historically we reported shared memory that way, and
> when it wasn't accurately meaning that anymore a 0 was hardcoded there to
> potentially not break compatibility with older tools (older than 2.4).
> In 2.6 we got rid of meminfo's "MemShared" until 2009, when you sort of
> re-introduced it re-branded as Shmem. IMO, we should leverage what we
> have in kernel now and take this change to make the exposed data consistent
> across the interfaces that export it today -- sysinfo(2) & /proc/meminfo.
>
> This is not a hard requirement, though, but rather a simple maintenance
> nitpick from code review.

Ok, ack then. But please update a patch description and repost w/
ccing linux-api@...r.kernel.org. Someone might have a specific concern
about a compatibility.
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