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

[Juergen Gross <jgross@suse.com>]

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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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