Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

From: Daniel Vetter <daniel@ffwll.ch>
To: John Hubbard <jhubbard@nvidia.com>
Cc: linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	Jan Kara <jack@suse.cz>, KVM list <kvm@vger.kernel.org>,
	Pawel Osciak <pawel@osciak.com>, Linux MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Tomasz Figa <tfiga@chromium.org>,
	Christoph Hellwig <hch@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	J??r??me Glisse <jglisse@redhat.com>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>,
	Daniel Vetter <daniel.vetter@intel.com>,
	Kyungmin Park <kyungmin.park@samsung.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
Date: Fri, 6 Nov 2020 11:01:57 +0100	[thread overview]
Message-ID: <CAKMK7uGRw=xXE+D=JJsNeRav9+hdO4tcDSvDbAuWfc3T4VkoJw@mail.gmail.com> (raw)
In-Reply-To: <7ae3486d-095e-cf4e-6b0f-339d99709996@nvidia.com>

On Fri, Nov 6, 2020 at 5:08 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 11/5/20 4:49 AM, Jason Gunthorpe wrote:
> > On Thu, Nov 05, 2020 at 10:25:24AM +0100, Daniel Vetter wrote:
> >>> /*
> >>>   * If we can't determine whether or not a pte is special, then fail immediately
> >>>   * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
> >>>   * to be special.
> >>>   *
> >>>   * For a futex to be placed on a THP tail page, get_futex_key requires a
> >>>   * get_user_pages_fast_only implementation that can pin pages. Thus it's still
> >>>   * useful to have gup_huge_pmd even if we can't operate on ptes.
> >>>   */
> >>
> >> We support hugepage faults in gpu drivers since recently, and I'm not
> >> seeing a pud_mkhugespecial anywhere. So not sure this works, but probably
> >> just me missing something again.
> >
> > It means ioremap can't create an IO page PUD, it has to be broken up.
> >
> > Does ioremap even create anything larger than PTEs?

gpu drivers also tend to use vmf_insert_pfn* directly, so we can do
on-demand paging and move buffers around. From what I glanced for
lowest level we to the pte_mkspecial correctly (I think I convinced
myself that vm_insert_pfn does that), but for pud/pmd levels it seems
just yolo.

remap_pfn_range seems to indeed split down to pte level always.

>  From my reading, yes. See ioremap_try_huge_pmd().

The ioremap here shouldn't matter, since this is for kernel-internal
mappings. So that's all fine I think.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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