From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: "John Hubbard" <jhubbard@nvidia.com>,
"DRI Development" <dri-devel@lists.freedesktop.org>,
LKML <linux-kernel@vger.kernel.org>,
"Daniel Vetter" <daniel.vetter@intel.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Jérôme Glisse" <jglisse@redhat.com>, "Jan Kara" <jack@suse.cz>,
"Dan Williams" <dan.j.williams@intel.com>,
"Linux MM" <linux-mm@kvack.org>,
"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
"open list:DMA BUFFER SHARING FRAMEWORK"
<linux-media@vger.kernel.org>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM
Date: Sun, 4 Oct 2020 13:20:31 +0200 [thread overview]
Message-ID: <CAKMK7uFjGDiWMD+KeEFziY-nqWy_FtCpn0rekb70yVLaxzJHjA@mail.gmail.com> (raw)
In-Reply-To: <20201003232441.GO9916@ziepe.ca>
On Sun, Oct 4, 2020 at 1:24 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> 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@nvidia.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.
Yeah I think that works. I tried understanding gup.c code a bit more,
and it looks like FOLL_LONGTERM only works for the pup_fast variant
right now? All others seem to have this comment that it's somehow
incompatible with FAULT_FLAG_ALLOW_RETRY and daxfs. But grepping
around for that didn't show up anything, at least not nearby dax code.
For my understanding of all this, what's the hiccup there?
For plans, I only started this for a bit of my own learning, but I
think I'll respin with the following changes:
- convert exynos and habanalabs to pin_user_pages_fast directly,
instead of going through this frame-vector detour
- move the locking and convert get_vaddr_frames to pup_fast as Jason suggested
- hack up some truly gross rfc to plug the follow_pfn hole
Cheers, Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
next prev parent reply other threads:[~2020-10-04 11:20 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-02 17:53 [PATCH 1/2] mm/frame-vec: Drop gup_flags from get_vaddr_frames() Daniel Vetter
2020-10-02 17:53 ` [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM Daniel Vetter
2020-10-02 18:06 ` Jason Gunthorpe
2020-10-02 18:16 ` Daniel Vetter
2020-10-02 23:31 ` Jason Gunthorpe
2020-10-03 8:34 ` Oded Gabbay
2020-10-03 9:40 ` Daniel Vetter
2020-10-04 12:50 ` Jason Gunthorpe
2020-10-04 16:09 ` Daniel Vetter
2020-10-05 17:28 ` Jason Gunthorpe
2020-10-05 18:16 ` Daniel Vetter
2020-10-05 18:37 ` Jason Gunthorpe
2020-10-05 18:54 ` Daniel Vetter
2020-10-05 22:43 ` Daniel Vetter
2020-10-05 23:41 ` Jason Gunthorpe
2020-10-06 6:23 ` Daniel Vetter
2020-10-06 12:26 ` Jason Gunthorpe
2020-10-06 13:08 ` Daniel Vetter
2020-10-07 10:47 ` Marek Szyprowski
2020-10-07 12:01 ` Daniel Vetter
2020-10-07 12:33 ` Marek Szyprowski
2020-10-07 12:44 ` Jason Gunthorpe
2020-10-07 12:47 ` Tomasz Figa
2020-10-07 12:58 ` Daniel Vetter
2020-10-07 13:06 ` Jason Gunthorpe
2020-10-07 13:34 ` Tomasz Figa
2020-10-07 13:42 ` Jason Gunthorpe
2020-10-07 14:08 ` Daniel Vetter
2020-10-07 14:11 ` Tomasz Figa
2020-10-07 14:22 ` Daniel Vetter
2020-10-07 15:05 ` Tomasz Figa
2020-10-07 14:58 ` Jason Gunthorpe
2020-10-07 13:06 ` Tomasz Figa
2020-10-07 13:14 ` Jason Gunthorpe
2020-10-05 15:03 ` Jan Kara
2020-10-02 22:39 ` John Hubbard
2020-10-03 9:45 ` Daniel Vetter
2020-10-03 22:52 ` John Hubbard
2020-10-03 23:24 ` Jason Gunthorpe
2020-10-04 11:20 ` Daniel Vetter [this message]
2020-10-05 17:35 ` Jason Gunthorpe
2020-10-02 18:22 ` [PATCH 1/2] mm/frame-vec: Drop gup_flags from get_vaddr_frames() Tomasz Figa
2020-10-02 19:21 ` Oded Gabbay
2020-10-05 17:38 [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM Jason Gunthorpe
2020-10-05 17:47 ` Jason Gunthorpe
2020-10-06 3:36 ` Andrew Morton
2020-10-06 11:57 ` Jason Gunthorpe
2020-10-05 17:53 ` Jan Kara
2020-10-05 17:57 ` Jason Gunthorpe
2020-10-05 18:16 ` Daniel Vetter
2020-10-06 11:56 ` Daniel Vetter
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