Re: [PATCH 5/6] iomap: implement direct I/O

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On Tue, Oct 25, 2016 at 09:13:29AM -0800, Kent Overstreet wrote:
> Also - what are you doing about the race between shooting down the range in the
> pagecache and dirty pages being readded? The existing direct IO code falls back
> to buffered IO for that, but your code doesn't appear to - I seem to recall that
> XFS has its own locking for this, are you just relying on that for now?  It'd be
> really nice to get some generic locking for this, anything that relies on
> pagecache invalidation is sketchy as hell in other filesystems.

Yes, XFS always had a shared/exclusive lock for I/O operations,
which is taken exclusive for buffered writes and those corner cases
of direct writes that needs exclusіon (e.g. sub-fs block size I/O).

This prevents new dirty pages from being added while direct I/O is
in progress.  There is nothing to prevent direct reads, though - that's
why both the old common code, the old XFS code and this new code do
a second invalidation after the write is done.

Now that the VFS i_mutex has been replaced with i_rwsem we can apply
this scheme to common code as well by taking i_rwsem shared for
direct I/O reads.
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