Re: [PATCH 6/7] nonblocking aio: xfs

[Christoph Hellwig <hch@infradead.org>]

On Wed, Feb 15, 2017 at 10:11:38AM -0600, Goldwyn Rodrigues wrote:
> > I did not understand short writes. Where can I
> > get a short write?

If you have a write request of N bytes, and you've already wrіtten
M of them you return M from the *write system call instead of -EAGAIN.
This is standard practice on e.g. sockets.

> > 
> > If I understand correctly, we do add the flag.
> 
> Replying to myself to correct myself.
> 
> On reading a bit more, I figured that we perform
> xfs_file_iomap_begin->xfs_iomap_write_direct. At this point we have
> already performed xfs_bmapi_read(). So, a check in xfs_file_iomap_begin
> should be good enough. So, the flag required would be with iomap flags,
> say IOMAP_NONBLOCKING. IOW, we don't need to go all the way to
> xfs_bmap_write() and return when imap.br_startblock == HOLESTARTBLOCK.

Yes, except that reflinked files with shared extents will also need
some additional special casing - for those xfs_bmapi_read can return
an allocated extent, but we might still have to perform a block
allocation for a write.