Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages

[Jan Kara <jack@suse.cz>]

On Mon 29-08-22 12:59:26, John Hubbard wrote:
> On 8/29/22 09:08, Jan Kara wrote:
> >> However, the core block/bio conversion in patch 4 still does depend upon
> >> a key assumption, which I got from a 2019 email discussion with
> >> Christoph Hellwig and others here [1], which says:
> >>
> >>     "All pages released by bio_release_pages should come from
> >>      get_get_user_pages...".
> >>
> >> I really hope that still holds true. Otherwise this whole thing is in
> >> trouble.
> >>
> >> [1] https://lore.kernel.org/kvm/20190724053053.GA18330@infradead.org/
> > 
> > Well as far as I've checked that discussion, Christoph was aware of pipe
> > pages etc. (i.e., bvecs) entering direct IO code. But he had some patches
> > [2] which enabled GUP to work for bvecs as well (using the kernel mapping
> > under the hood AFAICT from a quick glance at the series). I suppose we
> > could also handle this in __iov_iter_get_pages_alloc() by grabbing pin
> > reference instead of plain get_page() for the case of bvec iter. That way
> > we should have only pinned pages in bio_release_pages() even for the bvec
> > case.
> 
> OK, thanks, that looks viable. So, that approach assumes that the
> remaining two cases in __iov_iter_get_pages_alloc() will never end up
> being released via bio_release_pages():
> 
>     iov_iter_is_pipe(i)
>     iov_iter_is_xarray(i)
> 
> I'm actually a little worried about ITER_XARRAY, which is a recent addition.
> It seems to be used in ways that are similar to ITER_BVEC, and cephfs is
> using it. It's probably OK for now, for this series, which doesn't yet
> convert cephfs.

So after looking into that a bit more, I think a clean approach would be to
provide iov_iter_pin_pages2() and iov_iter_pages_alloc2(), under the hood
in __iov_iter_get_pages_alloc() make sure we use pin_user_page() instead of
get_page() in all the cases (using this in pipe_get_pages() and
iter_xarray_get_pages() is easy) and then make all bio handling use the
pinning variants for iters. I think at least iov_iter_is_pipe() case needs
to be handled as well because as I wrote above, pipe pages can enter direct
IO code e.g. for splice(2).

Also I think that all iov_iter_get_pages2() (or the _alloc2 variant) users
actually do want the "pin page" semantics in the end (they are accessing
page contents) so eventually we should convert them all to
iov_iter_pin_pages2() and remove iov_iter_get_pages2() altogether. But this
will take some more conversion work with networking etc. so I'd start with
converting bios only.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR