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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: Andrea Arcangeli <aarcange@redhat.com> Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Pavel Emelyanov <xemul@virtuozzo.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -381,8 +381,26 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
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* in __get_user_pages if userfaultfd_release waits on the
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* caller of handle_userfault to release the mmap_sem.
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*/
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if (unlikely(ACCESS_ONCE(ctx->released)))
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if (unlikely(ACCESS_ONCE(ctx->released))) {
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/*
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* Don't return VM_FAULT_SIGBUS in this case, so a non
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* cooperative manager can close the uffd after the
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* last UFFDIO_COPY, without risking to trigger an
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* involuntary SIGBUS if the process was starting the
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* userfaultfd while the userfaultfd was still armed
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* (but after the last UFFDIO_COPY). If the uffd
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* wasn't already closed when the userfault reached
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* this point, that would normally be solved by
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* userfaultfd_must_wait returning 'false'.
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*
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* If we were to return VM_FAULT_SIGBUS here, the non
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* cooperative manager would be instead forced to
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* always call UFFDIO_UNREGISTER before it can safely
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* close the uffd.
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*/
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ret = VM_FAULT_NOPAGE;
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goto out;
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}
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/*
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* Check that we can return VM_FAULT_RETRY.
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