Re: [PATCH v4] xen/gntdev: Avoid blocking in unmap_grant_pages()

[Juergen Gross <jgross@xxxxxxxx>]

On 22.06.22 04:27, 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.
>
> It is also necessary to guard against reentrancy in gntdev_map_put(),
> and to handle the case where userspace tries to map a mapping whose
> contents have not all been freed yet.
>
> Fixes: 745282256c75 ("xen/gntdev: safely unmap grants in case they are still in 
> use")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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