Re: [RFC PATCH 0/2] O_DIRECT locking rework

[Chris Mason <chris.mason@oracle.com>]

On Fri, Oct 20, 2006 at 01:03:24PM -0700, Zach Brown wrote:
> 
> > I think the fake placeholder page can be a kernel-wide thing rather than
> > per-dio?  That would be most desirable because then we have
> > 
> > #define PagePlaceHolder(page)	(page == global_placeholder_page)
> 
> That would be nice.  The current patch hashes the page pointer with
> page_waitqueue(page) to get at the zone's hashed wait queue buckets.  It
> would probably want to avoid that hot spot by getting at the buckets by
> hashing the mapping and file offset, I guess.

The page_waitqueue part only happens in the contention case.  This will
either be buffered vs direct io, direct io vs direct io or direct io vs
truncate.  I think all three should be rare enough that a single
waitqueue won't hurt.

It is also very hard to know the file offset without asking the radix
tree, since we can't trust placeholder_page->index.

-chris