linux-kernel - Re: [PATCH v3 12/16] PCI: Obey iomem restrictions for procfs mmap

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Date:   Wed, 21 Oct 2020 12:13:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, 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>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.com>
Subject: Re: [PATCH v3 12/16] PCI: Obey iomem restrictions for procfs mmap

On Wed, Oct 21, 2020 at 04:42:11PM +0200, Daniel Vetter wrote:

> Uh yes. In drivers/gpu this isn't a problem because we only install
> ptes from the vm_ops->fault handler. So no races. And I don't think
> you can fix this otherwise through holding locks: mmap_sem we can't
> hold because before vma_link we don't even know which mm_struct is
> involved, so can't solve the race. Plus this would be worse that
> mm_take_all_locks used by mmu notifier. And the address_space
> i_mmap_lock is also no good since it's not held during the ->mmap
> callback, when we write the ptes. And the resource locks is even less
> useful, since we're not going to hold that at vma_link() time for
> sure.
> 
> Hence delaying the pte writes after the vma_link, which means ->fault
> time, looks like the only way to close this gap.

> Trouble is I have no idea how to do this cleanly ...

How about add a vm_ops callback 'install_pages'/'prefault_pages' ?

Call it after vm_link() - basically just move the remap_pfn, under
some other lock, into there.

Jason