mm: document mmu_notifier_invalidate_range_start_nonblock()

Document what mmu_notifier_invalidate_range_start_nonblock() is for. Also
add a __must_check annotation to signal that callers must bail out if a
notifier vetoes the operation.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jann Horn <[e-mail]>
Reviewed-by: Jason Gunthorpe <[e-mail]>
Reviewed-by: Alistair Popple <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mmu_notifier.h

···
 	lock_map_release(&__mmu_notifier_invalidate_range_start_map);
 }
 
-static inline int
+/*
+ * This version of mmu_notifier_invalidate_range_start() avoids blocking, but it
+ * can return an error if a notifier can't proceed without blocking, in which
+ * case you're not allowed to modify PTEs in the specified range.
+ *
+ * This is mainly intended for OOM handling.
+ */
+static inline int __must_check
 mmu_notifier_invalidate_range_start_nonblock(struct mmu_notifier_range *range)
 {
 	int ret = 0;