mm: add comment on storage key dirty bit semantics

Add comments that dirty bit in storage key gets set whenever page content
is changed.  Hopefully if someone will use this function, he'll have a
look at one of the two places where we comment on this.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 46823fb0e801..cf7e99a87c32 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1151,9 +1151,11 @@ void page_remove_rmap(struct page *page)
          * containing the swap entry, but page not yet written to swap.
          *
          * And we can skip it on file pages, so long as the filesystem
-         * participates in dirty tracking; but need to catch shm and tmpfs
-         * and ramfs pages which have been modified since creation by read
-         * fault.
+         * participates in dirty tracking (note that this is not only an
+         * optimization but also solves problems caused by dirty flag in
+         * storage key getting set by a write from inside kernel); but need to
+         * catch shm and tmpfs and ramfs pages which have been modified since
+         * creation by read fault.
          *
          * Note that mapping must be decided above, before decrementing
          * mapcount (which luckily provides a barrier): once page is unmapped,