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Date: Tue, 8 Dec 2015 12:22:25 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [Intel-gfx] [PATCH v2 1/2] mm: Export nr_swap_pages
Message-ID: <20151208112225.GB25800@dhcp22.suse.cz>
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 <20151207134812.GA20782@dhcp22.suse.cz>
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To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Gordon <david.s.gordon@intel.com>, linux-mm@kvack.org, intel-gfx@lists.freedesktop.org, "Goel, Akash" <akash.goel@intel.com>

On Mon 07-12-15 14:13:46, Johannes Weiner wrote:
> On Mon, Dec 07, 2015 at 06:10:00PM +0000, Dave Gordon wrote:
> > Exporting random uncontrolled variables from the kernel to loaded modules is
> > not really considered best practice. It would be preferable to provide an
> > accessor function - which is just what the declaration says we have; the
> > implementation as a static inline (and/or macro) is what causes the problem
> > here.
> 
> No, what causes the problem is thinking we can't trust in-kernel code.

This is not about the trust. It is about a clear API and separation.

> If somebody screws up, we can fix it easily enough. Sure, we shouldn't
> be laying traps and create easy-to-misuse interfaces, but that's not
> what's happening here. There is no reason to add function overhead to
> what should be a single 'mov' instruction.

The mere fact that the current implementation is a simple atomic_long_read
is a detail and not important for the API. The function is not used
in any hot path where a single function call overhead would be a
performance killer. Exporting implementation details to random users
tends to add maintenance burden in future.

I think it is natural to export symbols which are consumed by modules
and that will be get_nr_swap_pages(). I do not even understand the
resistance against that. Anyway I am not going to argue about it more.
I have raised my review comment and leave the decision to Chris/Andrew.
-- 
Michal Hocko
SUSE Labs

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From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v2 1/2] mm: Export nr_swap_pages
Date: Tue, 8 Dec 2015 12:22:25 +0100
Message-ID: <20151208112225.GB25800@dhcp22.suse.cz>
References: <1449244734-25733-1-git-send-email-chris@chris-wilson.co.uk>
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