On Wed, Jan 13, 2021 at 10:29:23AM +1100, Dave Chinner wrote: > On Tue, Jan 12, 2021 at 05:26:15PM +0100, Christoph Hellwig wrote: > > Add a flag to request that the iomap instances do not allocate blocks > > by translating it to another new IOMAP_NOALLOC flag. > > Except "no allocation" that is not what XFS needs for concurrent > sub-block DIO. Well, this is just a quick draft. I could not come up with a better name, so I picked on that explains most but not all of what is going on. > If we're going to use a flag for this specific functionality, let's > call it what it is: IOMAP_DIO_UNALIGNED/IOMAP_UNALIGNED and do two > things with it. Sounds fine with me. > > 1. Make unaligned IO a formal part of the iomap_dio_rw() > behaviour so it can do the common checks to for things that > need exclusive serialisation for unaligned IO (i.e. avoid IO > spanning EOF, abort if there are cached pages over the > range, etc). Note that these all writes already fall back to buffered I/O if invalidate_inode_pages2_range fails, so there must never be cached pages for direct I/O these days. This is different from NOWAIT I/O where we simply give up if there are any cached pages and don't even try to invalidate them. > 2. require the filesystem mapping callback do only allow > unaligned IO into ranges that are contiguous and don't > require mapping state changes or sub-block zeroing to be > performed during the sub-block IO. Yeah.
