On 15:05 10/06, Dave Chinner wrote: > On Wed, Jun 10, 2020 at 12:59:00PM +1000, Dave Chinner wrote: > > [ Please cc the XFS list on XFS and iomap infrastructure changes.] > > > > On Fri, Jun 05, 2020 at 03:48:35PM -0500, Goldwyn Rodrigues wrote: > > > In current scenarios, for XFS, it would mean that a page invalidation > > > would end up being a writeback error. So, if iomap returns zero, fall > > > back to biffered I/O. XFS has never supported fallback to buffered I/O. > > > I hope it is not "never will" ;) > > > > I wouldn't say "never", but we are not going to change XFS behaviour > > because btrfs has a page invalidation vs DIO bug in it... > > Let me point out a specific "oh shit, I didn't think of that" sort > of problem that your blind fallback to buffered IO causes. Do this > via direct IO: > > pwritev2(RWF_NOWAIT) > > now have it fail invalidation in the direct IO path and fallback to > buffered write. What does buffered write do with it? > > > if (iocb->ki_flags & IOCB_NOWAIT) > return -EOPNOTSUPP; > > Yup, userspace gets a completely spurious and bogus -EOPNOTSUPP > error to pwritev2() because some 3rd party is accessing the same > file via mmap or buffered IO. > Oh shit, I didn't think about that! I think adding a flag to iomap_dio_rw() to return in case of page invalidation failure is the best option for now. -- Goldwyn
