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

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Message-ID: <20201026235903.GE1523783@nvidia.com>
Date:   Mon, 26 Oct 2020 20:59:03 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     John Hubbard <jhubbard@...dia.com>
CC:     <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, Oct 23, 2020 at 09:44:17PM -0700, John Hubbard wrote:
> > Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> >   mm/gup.c | 88 +++++++++++++++++++++++++++++---------------------------
> >   1 file changed, 46 insertions(+), 42 deletions(-)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 102877ed77a4b4..ecbe1639ea2af7 100644
> > +++ b/mm/gup.c
> > @@ -2671,13 +2671,42 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
> >   	return ret;
> >   }
> > +static unsigned int lockless_pages_from_mm(unsigned long addr,
> 
> It would be slightly more consistent to use "start" here, too, instead of addr.
> 
> Separately, I'm not joyful about the change to unsigned int for the
> return type. I understand why you did it and that's perfectly sound
> reasoning: there is no -ERRNO possible here, and nr_pinned will always
> be >=0. And it's correct, although it does have a type mismatch in the
> return value.

I did it because I had to check that ignoring a negative return or
doing some wonky negative arithmetic wasn't some sneaky beahvior. It
isn't, the value is really unsigned. So I documented it to save the
next person this work.

I think the proper response is to ultimately change the
gup_pgd_range() call tree to take the unsigned as well.

> a) change all the nr_pages and nr_pinned throughout, to "long", or
> 
> b) change all the nr_pages and nr_pinned all function args, and use int
> return types throughout, as a "O or -ERRNO, only" return value
> convention.

The gup_pgd_range() this stuff largely does return

I think gup_pgd_range() works as it does due to 
 
> > +	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;

I actually deliberately put it inside the if because there is nothing
tricky about ret < 0, that is basically perfectly normal. It is only
the logic to drop the error code sometimes that is tricky..

> 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.

It seems like a good idea to me

I'll get the other notes in a v2

Thanks,
Jason