[RFC PATCH 0 of 8] O_DIRECT locking rework

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

Hello everyone,

Here's a respin of the O_DIRECT locking changes.  There are some minor
updates, mostly cleanups based on suggestions from Zach Brown and Dave
Chinner.

Linus found a deadlock between the placeholders and the page fault handler
when userland uses an mmap of the file as the O_DIRECT buffer.  I'm
getting around that by capping placeholder extents at DIO_PAGES (64)
intervals when the file has mappings.  This matches the interval used by
get_user_pages and makes sure get_user_pages has been called on a given
range in the address space before placeholders are added.

I also hit a stale data exposure when get_block returns data past the
end of the placeholder extent.  A concurrent readpage could come in and
find metadata before the data had gotten to disk.  The fix is to
only ask get_block for data up to the end of the placeholder.

Generally, this isn't a problem because the common case is for
a single placeholder to cover the entire DIO.  So the suboptimal
allocation result wouldn't happen very often.

-chris