mm: describe mmap_sem rules for __lock_page_or_retry() and callers

Add a comment describing the circumstances in which
__lock_page_or_retry() will or will not release the mmap_sem when
returning 0.

Add comments to lock_page_or_retry()'s callers (filemap_fault(),
do_swap_page()) noting the impact on VM_FAULT_RETRY returns.

Add comments on up the call tree, particularly replacing the false "We
return with mmap_sem still held" comments.

Signed-off-by: Paul Cassella <cassella@cray.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e1474ae18c88..3df8c7db7a4e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -484,6 +484,9 @@ static inline int lock_page_killable(struct page *page)
 /*
  * lock_page_or_retry - Lock the page, unless this would block and the
  * caller indicated that it can handle a retry.
+ *
+ * Return value and mmap_sem implications depend on flags; see
+ * __lock_page_or_retry().
  */
 static inline int lock_page_or_retry(struct page *page, struct mm_struct *mm,
                                      unsigned int flags)