linux-kernel - Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL

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Date:   Fri, 6 Nov 2020 08:55:05 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Thomas Hellstrom <thomas.hellstrom@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        J??r??me Glisse <jglisse@...hat.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Pawel Osciak <pawel@...iak.com>,
        KVM list <kvm@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Linux MM <linux-mm@...ck.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

On Fri, Nov 06, 2020 at 11:27:59AM +0100, Daniel Vetter wrote:
> On Fri, Nov 6, 2020 at 11:01 AM Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > On Fri, Nov 6, 2020 at 5:08 AM John Hubbard <jhubbard@...dia.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.
> 
> So I dug around a bit more and ttm sets PFN_DEV | PFN_MAP to get past
> the various pft_t_devmap checks (see e.g. vmf_insert_pfn_pmd_prot()).
> x86-64 has ARCH_HAS_PTE_DEVMAP, and gup.c seems to handle these
> specially, but frankly I got totally lost in what this does.

The fact vmf_insert_pfn_pmd_prot() has all those BUG_ON's to prevent
putting VM_PFNMAP pages into the page tables seems like a big red
flag.

The comment seems to confirm what we are talking about here:

	/*
	 * If we had pmd_special, we could avoid all these restrictions,
	 * but we need to be consistent with PTEs and architectures that
	 * can't support a 'special' bit.
	 */

ie without the ability to mark special we can't block fast gup and
anyone who does O_DIRECT on these ranges will crash the kernel when it
tries to convert a IO page into a struct page.

Should be easy enough to directly test?

Putting non-struct page PTEs into a VMA without setting VM_PFNMAP just
seems horribly wrong to me.

Jason