linux-kernel - Re: [PATCH 2/2] mm/frame-vec: use FOLL

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Message-ID: <20201003232441.GO9916@ziepe.ca>
Date:   Sat, 3 Oct 2020 20:24:41 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

On Sat, Oct 03, 2020 at 03:52:32PM -0700, John Hubbard wrote:
> On 10/3/20 2:45 AM, Daniel Vetter wrote:
> > On Sat, Oct 3, 2020 at 12:39 AM John Hubbard <jhubbard@...dia.com> wrote:
> > > 
> > > On 10/2/20 10:53 AM, Daniel Vetter wrote:
> > > > For $reasons I've stumbled over this code and I'm not sure the change
> > > > to the new gup functions in 55a650c35fea ("mm/gup: frame_vector:
> > > > convert get_user_pages() --> pin_user_pages()") was entirely correct.
> > > > 
> > > > This here is used for long term buffers (not just quick I/O) like
> > > > RDMA, and John notes this in his patch. But I thought the rule for
> > > > these is that they need to add FOLL_LONGTERM, which John's patch
> > > > didn't do.
> > > 
> > > Yep. The earlier gup --> pup conversion patches were intended to not
> > > have any noticeable behavior changes, and FOLL_LONGTERM, with it's
> > > special cases and such, added some risk that I wasn't ready to take
> > > on yet. Also, FOLL_LONGTERM rules are only *recently* getting firmed
> > > up. So there was some doubt at least in my mind, about which sites
> > > should have it.
> > > 
> > > But now that we're here, I think it's really good that you've brought
> > > this up. It's definitely time to add FOLL_LONGTERM wherever it's missing.
> > 
> > So should I keep this patch, or will it collide with a series you're working on?
> 
> It doesn't collide with anything on my end yet, because I've been slow to
> pick up on the need for changing callsites to add FOLL_LONGTERM. :)
> 
> And it looks like that's actually a problem, because:
> 
> > 
> > Also with the firmed up rules, correct that I can also drop the
> > vma_is_fsdax check when the FOLL_LONGTERM flag is set?
> 
> That's the right direction to go *in general*, but I see that the
> pin_user_pages code is still a bit stuck in the past. And this patch
> won't actually work, with or without that vma_is_fsdax() check.
> Because:
> 
> get_vaddr_frames(FOLL_LONGTERM)
>    pin_user_pages_locked()
> 	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> 		return -EINVAL;

There is no particular reason this code needs to have the mm sem at
that point.

It should call pin_user_pages_fast() and only if that fails get the mmap
lock and extract the VMA to do broken hackery.

Jason