Re: [PATCH] iommu/vt-d: Fix mm refcounting to hold mm_count not mm_users

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2016-01-13 at 21:35 +0000, David Woodhouse wrote:
> Holding mm_users works OK for graphics, which was the first user of SVM
> with VT-d. However, it works less well for other devices, where we actually
> do a mmap() from the file descriptor to which the SVM PASID state is tied.
> 
> In this case on process exit we end up with a recursive reference count:
>  - The MM remains alive until the file is closed and the driver's release()
>    call ends up unbinding the PASID.
>  - The VMA corresponding to the mmap() remains intact until the MM is
>    destroyed.
>  - Thus the file isn't closed, even when exit_files() runs, because the
>    VMA is still holding a reference to it. And the MM remains alive…
> 
> To address this issue, we *stop* holding mm_users while the PASID is bound.
> We already hold mm_count by virtue of the MMU notifier, and that can be
> made to be sufficient.
> 
> It means that for a period during process exit, the fun part of mmput()
> has happened and exit_mmap() has been called so the MM is basically
> defunct. But the PGD still exists and the PASID is still bound to it.
> 
> During this period, we have to be very careful — exit_mmap() doesn't use
> mm->mmap_sem because it doesn't expect anyone else to be touching the MM
> (quite reasonably, since mm_users is zero). So we also need to fix the
> fault handler to just report failure if mm_users is already zero, and to
> temporarily bump mm_users while handling any faults.
> 
> Additionally, exit_mmap() calls mmu_notifier_release() *before* it tears
> down the page tables, which is too early for us to flush the IOTLB for
> this PASID. And __mmu_notifier_release() removes every notifier from the
> list, so when exit_mmap() finally *does* tear down the mappings and
> clear the page tables, we don't get notified. So we work around this by
> clearing the PASID table entry in our MMU notifier release() callback.
> That way, the hardware *can't* get any pages back from the page tables
> before they get cleared.
> 
> Hardware designers have confirmed that the resulting 'PASID not present'
> faults should be handled just as gracefully as 'page not present' faults,
> the important criterion being that they don't perturb the operation for
> any *other* PASID in the system.
> 
> Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
> Cc: stable@vger.kernel.org

OK, I've finally managed to test this on SKL hardware, having brought
the i915 SVM support up to date:
http://git.infradead.org/users/dwmw2/linux-svm.git/shortlog/refs/heads/i915-svm

I believe it's working — CQ can you confirm that you're still happy
with it, please? I'll send it to Linus if so.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation


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