Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5

[Peter Zijlstra]

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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