Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()

[Al Viro]

On Tue, Aug 25, 2020 at 02:54:28AM +0100, Al Viro wrote:
> On Fri, Aug 21, 2020 at 09:20:54PM -0700, John Hubbard wrote:
> 
> > Direct IO behavior:
> > 
> >     ITER_IOVEC:
> >         pin_user_pages_fast();
> >         break;
> > 
> >     ITER_KVEC:    // already elevated page refcount, leave alone
> >     ITER_BVEC:    // already elevated page refcount, leave alone
> >     ITER_PIPE:    // just, no :)
> 
> Why?  What's wrong with splice to O_DIRECT file?

Sorry - s/to/from/, obviously.

To spell it out: consider generic_file_splice_read() behaviour when
the source had been opened with O_DIRECT; you will get a call of
->read_iter() into ITER_PIPE destination.  And it bloody well
will hit iov_iter_get_pages() on common filesystems, to pick the
pages we want to read into.

So... what's wrong with having that "pin" primitive making sure
the pages are there and referenced by the pipe?