On Tue, May 22, 2018 at 10:07:45AM +1000, Dave Chinner wrote: > > Something doesn't smell right here. The only pages we need to read in > > are the first and last pages in the write_begin range, and only if they > > aren't page aligned and the underlying extent is IOMAP_MAPPED, right? > > And not beyond EOF, too. > > The bufferhead code handles this via the buffer_new() flag - it > triggers the skipping of read IO and the states in which it is > set are clearly indicated in iomap_to_bh(). That same logic needs to > apply here. The buffer_new logic itself isn't really something to copy directly as it has all kinds of warts.. > > I also noticed that speculative preallocation kicks in by the second 80M > > write() call and writeback for the second call can successfully allocate > > the entire preallocation, which means that the third (or nth) write call > > can have a real extent already mapped in, and then we end up reading it. > > Yeah, that's because there's no check against EOF here. These writes > are all beyond EOF, so there shouldn't be any read at all... The EOF case is already handled in iomap_block_needs_zeroing. We just need to skip the read for ranges entirely covered by the write.
