linux-kernel - Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock

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Message-ID: <20140521213354.GL2485@laptop.programming.kicks-ass.net>
Date:	Wed, 21 May 2014 23:33:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Oleg Nesterov <oleg@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue
 lookups in unlock_page fastpath v5

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..
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