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Date: Wed, 21 May 2014 23:33:54 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>, Oleg Nesterov <oleg@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Vlastimil Babka <vbabka@suse.cz>,
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        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Howells <dhowells@redhat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue
 lookups in unlock_page fastpath v5
Message-ID: <20140521213354.GL2485@laptop.programming.kicks-ass.net>
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On Wed, May 21, 2014 at 02:26:22PM -0700, Andrew Morton wrote:
> > +static inline void
> > +__prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait,
> > +			struct page *page, int state, bool exclusive)
> 
> Putting MM stuff into core waitqueue code is rather bad.  I really
> don't know how I'm going to explain this to my family.

Right, so we could avoid all that and make the functions in mm/filemap.c
rather large and opencode a bunch of wait.c stuff.

Which is pretty much what I initially pseudo proposed.

> > +		__ClearPageWaiters(page);
> 
> We're freeing the page - if someone is still waiting on it then we have
> a huge bug?  It's the mysterious collision thing again I hope?

Yeah, so we only clear that bit when at 'unlock' we find there are no
more pending waiters, so if the last unlock still had a waiter, we'll
leave the bit set. So its entirely reasonable to still have it set when
we free a page etc..