netdev - Re: [PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL

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Date:   Wed, 13 Nov 2019 10:31:38 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL_PIN

> > +/**
> > + * pin_user_pages_fast() - pin user pages in memory without taking locks
> > + *
> > + * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
> > + * get_user_pages_fast() for documentation on the function arguments, because
> > + * the arguments here are identical.
> > + *
> > + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> > + * see Documentation/vm/pin_user_pages.rst for further details.
> > + *
> > + * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
> > + * is NOT intended for Case 2 (RDMA: long-term pins).
> > + */
> > +int pin_user_pages_fast(unsigned long start, int nr_pages,
> > +			unsigned int gup_flags, struct page **pages)
> > +{
> > +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> > +		return -EINVAL;
> > +
> > +	gup_flags |= FOLL_PIN;
> > +	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> > +}
> > +EXPORT_SYMBOL_GPL(pin_user_pages_fast);
> 
> I was somewhat wondering about the number of functions you add here. So we
> have:
> 
> pin_user_pages()
> pin_user_pages_fast()
> pin_user_pages_remote()
> 
> and then longterm variants:
> 
> pin_longterm_pages()
> pin_longterm_pages_fast()
> pin_longterm_pages_remote()
> 
> and obviously we have gup like:
> get_user_pages()
> get_user_pages_fast()
> get_user_pages_remote()
> ... and some other gup variants ...
> 
> I think we really should have pin_* vs get_* variants as they are very
> different in terms of guarantees and after conversion, any use of get_*
> variant in non-mm code should be closely scrutinized. OTOH pin_longterm_*
> don't look *that* useful to me and just using pin_* instead with
> FOLL_LONGTERM flag would look OK to me and somewhat reduce the number of
> functions which is already large enough? What do people think? I don't feel
> too strongly about this but wanted to bring this up.

I'm a bit concerned with the function explosion myself.  I think what you
suggest is a happy medium.  So I'd be ok with that.

Ira