linux-kernel - Re: [PATCH 0/2] Move away from non-failing small allocations

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Date:	Mon, 16 Mar 2015 15:38:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Dave Chinner <david@...morbit.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 0/2] Move away from non-failing small allocations

On Wed, 11 Mar 2015 16:54:52 -0400 Michal Hocko <mhocko@...e.cz> wrote:

> as per discussion at LSF/MM summit few days back it seems there is a
> general agreement on moving away from "small allocations do not fail"
> concept.

Such a change affects basically every part of the kernel and every
kernel developer.  I expect most developers will say "it works well
enough and I'm not getting any bug reports so why should I spend time
on this?".  It would help if we were to explain the justification very
clearly.  https://lwn.net/Articles/636017/ is Jon's writeup of the
conference discussion.

Realistically, I don't think this overall effort will be successful -
we'll add the knob, it won't get enough testing and any attempt to
alter the default will be us deliberately destabilizing the kernel
without knowing how badly :(

I wonder if we can alter the behaviour only for filesystem code, so we
constrain the new behaviour just to that code where we're having
problems.  Most/all fs code goes via vfs methods so there's a reasonably
small set of places where we can call

static inline void enter_fs_code(struct super_block *sb)
{
	if (sb->my_small_allocations_can_fail)
		current->small_allocations_can_fail++;
}

that way (or something similar) we can select the behaviour on a per-fs
basis and the rest of the kernel remains unaffected.  Other subsystems
can opt in as well.

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