Re: [PATCH v12 01/10] vfs, iomap: Fix generic_file_splice_read() to avoid reversion of ITER_PIPE

[Christoph Hellwig]

Subject nitpick:  this does not ouch iomap at all.

> Fix this by replacing the use of an ITER_PIPE iterator with an ITER_BVEC
> iterator for which reversion won't free the buffers.  Bulk allocate all the
> buffers we think we're going to use in advance, do the read synchronously
> and only then trim the buffer down.  The pages we did use get pushed into
> the pipe.
> 
> This is more efficient by virtue of doing a bulk page allocation, but
> slightly less efficient by ignoring any partial page in the pipe.

For the usual case of a buffered read into the iter, this completely
changes the semantics:

 - before the pagecache pages just grew a new reference and were
   added to the pipe buffer, and I/O was done without an extra
   copy
 - with this patch you always allocate an extra set of pages for
   the pipe and copy to it

So I very much doubt this makes anything more efficient, and I don't
think we can just do it.

We'll have to fix reverting of pipe buffers, just as I already pointed
out in your cifs series that tries to play the same game.