Re: [PATCH 1/2] mm: reorganize internal_get_user_pages_fast()

Jan Kara
2020-10-27T09:33:04Z

On Fri 23-10-20 21:44:17, John Hubbard wrote:
> On 10/23/20 5:19 PM, Jason Gunthorpe wrote:
> > + start += (unsigned long)nr_pinned << PAGE_SHIFT;
> > + pages += nr_pinned;
> > + ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
> > + pages);
> > + if (ret < 0) {
> > /* Have to be a bit careful with return values */
>
> ...and can we move that comment up one level, so that it reads:
>
> /* Have to be a bit careful with return values */
> if (ret < 0) {
> if (nr_pinned)
> return nr_pinned;
> return ret;
> }
> return ret + nr_pinned;
>
> Thinking about this longer term, it would be nice if the whole gup/pup API
> set just stopped pretending that anyone cares about partial success, because
> they *don't*. If we had return values of "0 or -ERRNO" throughout, and an
> additional set of API wrappers that did some sort of limited retry just like
> some of the callers do, that would be a happier story.

Actually there are callers that care about partial success. See e.g.
iov_iter_get_pages() usage in fs/direct_io.c:dio_refill_pages() or
bio_iov_iter_get_pages(). These places handle partial success just fine and
not allowing partial success from GUP could regress things...

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