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Date:   Wed, 7 Oct 2020 20:24:48 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        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>,
        "Linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>, Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci@vger.kernel.org
Subject: Re: [PATCH 10/13] PCI: revoke mappings like devmem
Message-ID: <20201007232448.GC5177@ziepe.ca>
References: <20201007164426.1812530-1-daniel.vetter@ffwll.ch>
 <20201007164426.1812530-11-daniel.vetter@ffwll.ch>
 <CAPcyv4hBL68A7CZa+YnooufDH2tevoxrx32DTJMQ6OHRnec7QQ@mail.gmail.com>
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On Wed, Oct 07, 2020 at 12:33:06PM -0700, Dan Williams wrote:
> On Wed, Oct 7, 2020 at 11:11 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> >
> > Since 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims
> > the region") /dev/kmem zaps ptes when the kernel requests exclusive
> > acccess to an iomem region. And with CONFIG_IO_STRICT_DEVMEM, this is
> > the default for all driver uses.
> >
> > Except there's two more ways to access pci bars: sysfs and proc mmap
> > support. Let's plug that hole.
> 
> Ooh, yes, lets.
> 
> >
> > For revoke_devmem() to work we need to link our vma into the same
> > address_space, with consistent vma->vm_pgoff. ->pgoff is already
> > adjusted, because that's how (io_)remap_pfn_range works, but for the
> > mapping we need to adjust vma->vm_file->f_mapping. Usually that's done
> > at ->open time, but that's a bit tricky here with all the entry points
> > and arch code. So instead create a fake file and adjust vma->vm_file.
> 
> I don't think you want to share the devmem inode for this, this should
> be based off the sysfs inode which I believe there is already only one
> instance per resource. In contrast /dev/mem can have multiple inodes
> because anyone can just mknod a new character device file, the same
> problem does not exist for sysfs.

The inode does not come from the filesystem char/mem.c creates a
singular anon inode in devmem_init_inode()

Seems OK to use this more widely, but it feels a bit weird to live in
char/memory.c.

This is what got me thinking maybe this needs to be a bit bigger
generic infrastructure - eg enter this scheme from fops mmap and
everything else is in mm/user_iomem.c

Jason