On 8/31/22 02:43, Jan Kara wrote:

quoted

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).

OK, yes.

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

I'll give this approach a spin, thanks very much for the guidance!


thanks,

-- 
John Hubbard
NVIDIA