On Wed, Jan 13, 2021 at 10:32:15AM -0500, Brian Foster wrote:

Something I hadn't thought about before is whether applications might
depend on current unaligned dio serialization for coherency and thus
break if the kernel suddenly allows concurrent unaligned dio to pass
through. Should this be something that is explicitly requested by
userspace?

direct I/O has always been documented as not being synchronized.  Also
for block devices you won't get any synchronization at all, down to
the sector level.

That aside, I agree that the DIO_UNALIGNED approach seems a bit more
clear than NOALLOC, but TBH the more I look at this the more Christoph's
first approach seems cleanest to me. It is a bit unfortunate to
duplicate the mapping lookups and have the extra ILOCK cycle, but the
lock is shared and only taken when I/O is unaligned. I don't really see
why that is a show stopper yet it's acceptable to fall back to exclusive
dio if the target range happens to be discontiguous (but otherwise
mapped/written).

I think both approaches have pros an cons.  My original one (which really
is yours as you suggested it) has the advantage of having a much simpler
structure, and not limititing the non-exclusive I/O to a single extent.
The refined version of Dave's approach avoids the extra one or two extent
lookups, and any knowledge of extent state above the iomap layer.