Re: [PATCH 5.10] xen/gntdev: Avoid blocking in unmap_grant_pages()

[Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>]

On Mon, Jun 27, 2022 at 02:10:02PM -0400, Demi Marie Obenour wrote:
> commit dbe97cff7dd9f0f75c524afdd55ad46be3d15295 upstream
> 
> 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>
> Link: 
> https://lore.kernel.org/r/20220622022726.2538-1-demi@xxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  drivers/xen/gntdev-common.h |   7 ++
>  drivers/xen/gntdev.c        | 142 +++++++++++++++++++++++++-----------
>  2 files changed, 106 insertions(+), 43 deletions(-)

All now queued up, thanks.

greg k-h