linux-kernel - Re: [PATCH 06/10] mm, page_alloc: Rename __GFP_WAIT to __GFP

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Date:	Mon, 28 Sep 2015 16:55:23 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/10] mm, page_alloc: Rename __GFP_WAIT to
 __GFP_RECLAIM

On Mon, 21 Sep 2015 11:52:38 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:

> __GFP_WAIT was used to signal that the caller was in atomic context and
> could not sleep.  Now it is possible to distinguish between true atomic
> context and callers that are not willing to sleep. The latter should clear
> __GFP_DIRECT_RECLAIM so kswapd will still wake. As clearing __GFP_WAIT
> behaves differently, there is a risk that people will clear the wrong
> flags. This patch renames __GFP_WAIT to __GFP_RECLAIM to clearly indicate
> what it does -- setting it allows all reclaim activity, clearing them
> prevents it.

We have quite a history of remote parts of the kernel using
weird/wrong/inexplicable combinations of __GFP_ flags.  I tend to think
that this is because we didn't adequately explain the interface.

And I don't think that gfp.h really improved much in this area as a
result of this patchset.  Could you go through it some time and decide
if we've adequately documented all this stuff?

GFP_ATOMIC vs GFP_NOWAIT?

GFP_USER vs GFP_HIGHUSER?

When should I use GFP_HIGHUSER_MOVABLE instead?

Why isn't there a GFP_USER_MOVABLE?

What's GFP_IOFS?

GFP_RECLAIM_MASK through GFP_SLAB_BUG_MASK are mm-internal, but look
the same as the exported interface definitions.

__GFP_MOVABLE is documented twice, the second in an odd place.

etcetera.

It's rather unclear which symbols are part of the exported interface
and which are "mm internal only".
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