userfaultfd: non-cooperative: Add fork() event

userfaultfd: non-cooperative: Add fork() event

When the mm with uffd-ed vmas fork()-s the respective vmas notify their
uffds with the event which contains a descriptor with new uffd.  This
new descriptor can then be used to get events from the child and
populate its mm with data.  Note, that there can be different uffd-s
controlling different vmas within one mm, so first we should collect all
those uffds (and ctx-s) in a list and then notify them all one by one
but only once per fork().

The context is created at fork() time but the descriptor, file struct
and anon inode object is created at event read time.  So some trickery
is added to the userfaultfd_ctx_read() to handle the ctx queues' locking
vs file creation.

Another thing worth noticing is that the task that fork()-s waits for
the uffd event to get processed WITHOUT the mmap sem.

[[email protected]: build warning fix]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Pavel Emelyanov <[email protected]>
Signed-off-by: Mike Rapoport <[email protected]>
Signed-off-by: Andrea Arcangeli <[email protected]>
Cc: "Dr. David Alan Gilbert" <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Michael Rapoport <[email protected]>
Cc: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>