xen/gntdev: Avoid blocking in unmap_grant_pages()
stable inclusion from stable-v5.10.129 commit 79963021fd718b74bed4cbc98f5f49d3ba6fb48c category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I5YNDQ Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=79963021fd718b74bed4cbc98f5f49d3ba6fb48c -------------------------------- commit dbe97cff 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: 74528225 ("xen/gntdev: safely unmap grants in case they are still in use") Cc: stable@vger.kernel.org Signed-off-by:Demi Marie Obenour <demi@invisiblethingslab.com> Reviewed-by:
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