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| author | Andrea Arcangeli <aarcange@redhat.com> | 2017-09-08 16:12:42 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2017-09-08 18:26:47 -0700 |
| commit | 656710a60e3693911bee3a355d2f2bbae3faba33 (patch) | |
| tree | dd55fb4c43f3cf65c50d0795a001370429b4d6e0 /fs/userfaultfd.c | |
| parent | 98c70baad4067cf3bfb90c26b3b01cb3eab621e2 (diff) | |
| download | lwn-656710a60e3693911bee3a355d2f2bbae3faba33.tar.gz lwn-656710a60e3693911bee3a355d2f2bbae3faba33.zip | |
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>
Diffstat (limited to 'fs/userfaultfd.c')
| -rw-r--r-- | fs/userfaultfd.c | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 5419e7da82ba..ef4b48d1ea42 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -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. |