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Commit 656710a6 authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds
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userfaultfd: non-cooperative: closing the uffd without triggering SIGBUS 

This is an enhancement to avoid a non cooperative userfaultfd manager
having to unregister all regions before it can close the uffd after all
userfaultfd activity completed.

The UFFDIO_UNREGISTER would serialize against the handle_userfault by
taking the mmap_sem for writing, but we can simply repeat the page fault
if we detect the uffd was closed and so the regular page fault paths
should takeover.

Link: http://lkml.kernel.org/r/20170823181227.19926-1-aarcange@redhat.com


Signed-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
Acked-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 98c70baa

fs/userfaultfd.c

+19 −1
Original line number Diff line number Diff line
@@ -381,8 +381,26 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason) 
	 * in __get_user_pages if userfaultfd_release waits on the

	 * caller of handle_userfault to release the mmap_sem.

	 */

	if (unlikely(ACCESS_ONCE(ctx->released)))

	if (unlikely(ACCESS_ONCE(ctx->released))) {

		/*

		 * Don't return VM_FAULT_SIGBUS in this case, so a non

		 * cooperative manager can close the uffd after the

		 * last UFFDIO_COPY, without risking to trigger an

		 * involuntary SIGBUS if the process was starting the

		 * userfaultfd while the userfaultfd was still armed

		 * (but after the last UFFDIO_COPY). If the uffd

		 * wasn't already closed when the userfault reached

		 * this point, that would normally be solved by

		 * userfaultfd_must_wait returning 'false'.

		 *

		 * If we were to return VM_FAULT_SIGBUS here, the non

		 * cooperative manager would be instead forced to

		 * always call UFFDIO_UNREGISTER before it can safely

		 * close the uffd.

		 */

		ret = VM_FAULT_NOPAGE;

		goto out;

	}



	/*

	 * Check that we can return VM_FAULT_RETRY.