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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed, 21 Oct 2020 21:24:08 +0200
Message-ID: <CAKMK7uEjE5sHUq0hV_bnYjPKRxYyBnty0sLre+owANGZjLJg9Q@mail.gmail.com>
Subject: Re: [PATCH v3 12/16] PCI: Obey iomem restrictions for procfs mmap
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, 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>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>, Bjorn Helgaas <bhelgaas@google.com>,
        Linux PCI <linux-pci@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@ffwll.com>
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On Wed, Oct 21, 2020 at 6:37 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Wed, Oct 21, 2020 at 05:54:54PM +0200, Daniel Vetter wrote:
>
> > The trouble is that io_remap_pfn adjust vma->pgoff, so we'd need to
> > split that. So ideally ->mmap would never set up any ptes.
>
> /dev/mem makes pgoff == pfn so it doesn't get changed by remap.
>
> pgoff doesn't get touched for MAP_SHARED either, so there are other
> users that could work like this - eg anyone mmaping IO memory is
> probably OK.

I was more generally thinking for io_remap_pfn_users because of the
mkwrite use-case we might have in fbdev emulation in drm.

> > I guess one option would be if remap_pfn_range would steal the
> > vma->vm_ops pointer for itself, then it could set up the correct
> > ->install_ptes hook. But there's tons of callers for that, so not sure
> > that's a bright idea.
>
> The caller has to check that the mapping is still live, and I think
> hold a lock across the remap? Auto-defering it doesn't seem feasible.

Right auto-defering reopens the race, so making this work
automatically is a bit much. I guess just splitting this into a
setup/install part and then doing the install of all the ptes at first
fault should be good enough. We don't really need a new install_pages
for that, just an io_remap_pfn_range that's split in two parts.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch