netdev - Re: [patch 1/2] mm: remove GFP

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Date:	Thu, 26 Feb 2015 19:09:53 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	Pravin Shelar <pshelar@...ira.com>,
	Jarno Rajahalme <jrajahalme@...ira.com>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, netdev@...r.kernel.org, dev@...nvswitch.org
Subject: Re: [patch 1/2] mm: remove GFP_THISNODE

On Thu, 26 Feb 2015, Vlastimil Babka wrote:

> > NOTE: this is not about __GFP_THISNODE, this is only about GFP_THISNODE.
> > 
> > GFP_THISNODE is a secret combination of gfp bits that have different
> > behavior than expected.  It is a combination of __GFP_THISNODE,
> > __GFP_NORETRY, and __GFP_NOWARN and is special-cased in the page allocator
> > slowpath to fail without trying reclaim even though it may be used in
> > combination with __GFP_WAIT.
> > 
> > An example of the problem this creates: commit e97ca8e5b864 ("mm: fix 
> > GFP_THISNODE callers and clarify") fixed up many users of GFP_THISNODE
> > that really just wanted __GFP_THISNODE.  The problem doesn't end there,
> > however, because even it was a no-op for alloc_misplaced_dst_page(),
> > which also sets __GFP_NORETRY and __GFP_NOWARN, and
> > migrate_misplaced_transhuge_page(), where __GFP_NORETRY and __GFP_NOWAIT
> > is set in GFP_TRANSHUGE.  Converting GFP_THISNODE to __GFP_THISNODE is
> > a no-op in these cases since the page allocator special-cases
> > __GFP_THISNODE && __GFP_NORETRY && __GFP_NOWARN.
> > 
> > It's time to just remove GFP_TRANSHUGE entirely.  We leave __GFP_THISNODE
> 
>                               ^THISNODE :) Although yes, it would be nice if we
> could replace the GFP_TRANSHUGE magic checks as well.
> 

Haha, I referenced GFP_TRANSHUGE twice here when I meant GFP_THISNODE, I 
must want to fix that up as well.

> > to restrict an allocation to a local node, but remove GFP_TRANSHUGE and
> > it's obscurity.  Instead, we require that a caller clear __GFP_WAIT if it
> > wants to avoid reclaim.
> > 
> > This allows the aforementioned functions to actually reclaim as they
> > should.  It also enables any future callers that want to do
> > __GFP_THISNODE but also __GFP_NORETRY && __GFP_NOWARN to reclaim.  The
> > rule is simple: if you don't want to reclaim, then don't set __GFP_WAIT.
> 
> So, I agree with the intention, but this has some subtle implications that
> should be mentioned/decided. The check for GFP_THISNODE in
> __alloc_pages_slowpath() comes earlier than the check for __GFP_WAIT. So the
> differences will be:
> 
> 1) We will now call wake_all_kswapds(), unless __GFP_NO_KSWAPD is passed, which
> is only done for hugepages and some type of i915 allocation. Do we want the
> opportunistic attempts from slab to wake up kswapds or do we pass the flag?
> 
> 2) There will be another attempt on get_page_from_freelist() with different
> alloc_flags than in the fast path attempt. Without __GFP_WAIT (and also, again,
> __GFP_KSWAPD, since your commit b104a35d32, which is another subtle check for
> hugepage allocations btw), it will consider the allocation atomic and add
> ALLOC_HARDER flag, unless __GFP_NOMEMALLOC is in __gfp_flags - it seems it's
> generally not. It will also clear ALLOC_CPUSET, which was the concern of
> b104a35d32. However, if I look at __cpuset_node_allowed(), I see that it's
> always true for __GFP_THISNODE, which makes me question commit b104a35d32 in
> light of your patch 2/2 and generally the sanity of all these flags and my
> career choice.
> 

Do we do either of these?  gfp_exact_node() sets __GFP_THISNODE and clears 
__GFP_WAIT which will make the new conditional trigger immediately for 
NUMA configs.

Existing callers of GFP_KERNEL | __GFP_THISNODE aren't impacted and 
net/openvswitch/flow.c is mentioned in the changelog as actually wanting 
GFP_NOWAIT | __GFP_THISNODE so that this early check still fails.

> Ugh :)
> 

Ugh indeed.
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