Re: [PATCH 16/34] iomap: add initial support for writes without buffer heads

[Christoph Hellwig <hch@lst.de>]

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.