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>

diff --git a/arch/s390/include/asm/page.h b/arch/s390/include/asm/page.h
index 24afd079e5cee6cc331e6bf577e8f87e5b9e9662..a86ad4084073c85b2f6307f780d07336e470db5b 100644 (file)
--- a/arch/s390/include/asm/page.h
+++ b/arch/s390/include/asm/page.h
@@ -160,6 +160,9 @@ static inline int page_reset_referenced(unsigned long addr)
  * race against modification of the referenced bit. This function
  * should therefore only be called if it is not mapped in any
  * address space.
+ *
+ * Note that the bit gets set whenever page content is changed. That means
+ * also when the page is modified by DMA or from inside the kernel.
  */
 #define __HAVE_ARCH_PAGE_TEST_AND_CLEAR_DIRTY
 static inline int page_test_and_clear_dirty(unsigned long pfn, int mapped)

diff --git a/mm/rmap.c b/mm/rmap.c
index 46823fb0e801b599178f8bd548437fa03d87b8ef..cf7e99a87c32865c4e6cf5b6ed4f48f7f05aa9e0 100644 (file)
--- 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,