Re: [PATCH 09/10] iomap: add a IOMAP_DIO_NOALLOC flag

Christoph Hellwig
2021-01-14T17:30:14Z

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.