Re: [PATCH v2] xen/gntdev: Avoid blocking in unmap_grant_pages()

[Juergen Gross <jgross@xxxxxxxx>]

On 30.05.22 19:50, Demi Marie Obenour wrote:
> On Mon, May 30, 2022 at 08:41:20AM +0200, Juergen Gross wrote:
> > On 25.05.22 20:41, Demi Marie Obenour wrote:
> > > unmap_grant_pages() currently waits for the pages to no longer be used.
> > > In https://github.com/QubesOS/qubes-issues/issues/7481, this lead to a
> > > deadlock against i915: i915 was waiting for gntdev's MMU notifier to
> > > finish, while gntdev was waiting for i915 to free its pages.  I also
> > > believe this is responsible for various deadlocks I have experienced in
> > > the past.
> > >
> > > Avoid these problems by making unmap_grant_pages async.  This requires
> > > making it return void, as any errors will not be available when the
> > > function returns.  Fortunately, the only use of the return value is a
> > > WARN_ON(), which can be replaced by a WARN_ON when the error is
> > > detected.  Additionally, a failed call will not prevent further calls
> > > from being made, but this is harmless.
> > >
> > > Because unmap_grant_pages is now async, the grant handle will be sent to
> > > INVALID_GRANT_HANDLE too late to prevent multiple unmaps of the same
> > > handle.  Instead, a separate bool array is allocated for this purpose.
> > > This wastes memory, but stuffing this information in padding bytes is
> > > too fragile.  Furthermore, it is necessary to grab a reference to the
> > > map before making the asynchronous call, and release the reference when
> > > the call returns.
> > I think there is even more syncing needed:
> >
> > - In the error path of gntdev_mmap() unmap_grant_pages() is being called and
> >    it is assumed, map is available afterwards again. This should be rather easy
> >    to avoid by adding a counter of active mappings to struct gntdev_grant_map
> >    (number of pages not being unmapped yet). In case this counter is not zero
> >    gntdev_mmap() should bail out early.
> Is it possible to just unmap the pages directly here?  I don’t think
> there can be any other users of these pages yet.  Userspace could race
> against the unmap and cause a page fault, but that should just cause
> userspace to get SIGSEGV or SIGBUS.  In any case this code can use the
> sync version; if it gets blocked that’s userspace’s problem.
>
> > - gntdev_put_map() is calling unmap_grant_pages() in case the refcount has
> >    dropped to zero. This call can set the refcount to 1 again, so there is
> >    another delay needed before freeing map. I think unmap_grant_pages() should
> >    return in case the count of mapped pages is zero (see above), thus avoiding
> >    to increment the refcount of map if nothing is to be done. This would enable
> >    gntdev_put_map() to just return after the call of unmap_grant_pages() in case
> >    the refcount has been incremented again.
> I will change this in v3, but I do wonder if gntdev is using the wrong
> MMU notifier callback.  It seems that the appropriate callback is
> actually release(): if I understand correctly, release() is called
> precisely when the refcount on the physical page is about to drop to 0,
> and that is what we want.
No, I don't think this is correct.

release() is being called when the refcount of the address space is about
to drop to 0. It has no page granularity.


Juergen

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