mm/gup.c: further document vma_permits_fault()

Describe the caller's responsibilities when passing
FAULT_FLAG_ALLOW_RETRY.

Link: http://lkml.kernel.org/r/1586915606.5647.5.camel@mtkswgap22Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -1176,7 +1176,8 @@ static bool vma_permits_fault(struct vm_area_struct *vma,
  * @address:	user address
  * @fault_flags:flags to pass down to handle_mm_fault()
  * @unlocked:	did we unlock the mmap_sem while retrying, maybe NULL if caller
- *		does not allow retry
+ *		does not allow retry. If NULL, the caller must guarantee
+ *		that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY.
  *
  * This is meant to be called in the specific scenario where for locking reasons
  * we try to access user memory in atomic context (within a pagefault_disable()