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@mtkswgap22
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index c301466..1d84291 100644 (file)
--- a/mm/gup.c
+++ b/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()