linux-kernel - Re: [PATCH 1/2] mm: reorganize internal_get_user_pages

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Message-ID: <20201027093301.GA16090@quack2.suse.cz>
Date:   Tue, 27 Oct 2020 10:33:01 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Linux-MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Oleg Nesterov <oleg@...hat.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 1/2] mm: reorganize internal_get_user_pages_fast()

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@...e.com>
SUSE Labs, CR