[patch 03/39] add swap cache mapping comment

[blaisorblade@yahoo.it]

From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

Add some more comments about page->mapping and swapper_space, explaining their
(historical and current) relationship. Such material can be extracted from the
old GIT history (which I used for reference), but having it in the source is
more useful.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
---

 linux-2.6.git-paolo/./mm/swap_state.c |    5 +++++
 1 files changed, 5 insertions(+)

diff -puN ./mm/swap_state.c~swap-cache-mapping-comment ./mm/swap_state.c
--- linux-2.6.git/./mm/swap_state.c~swap-cache-mapping-comment	2005-08-11 11:12:57.000000000 +0200
+++ linux-2.6.git-paolo/./mm/swap_state.c	2005-08-11 11:12:57.000000000 +0200
@@ -21,6 +21,11 @@
  * swapper_space is a fiction, retained to simplify the path through
  * vmscan's shrink_list, to make sync_page look nicer, and to allow
  * future use of radix_tree tags in the swap cache.
+ *
+ * In 2.4 and until 2.6.6 pages in the swap cache also had page->mapping ==
+ * &swapper_space (this was the definition of PageSwapCache), but this is no
+ * more true. Instead, we use page->flags for that, and page->mapping is
+ * *ignored* here. However, also take a look at page_mapping().
  */
 static struct address_space_operations swap_aops = {
 	.writepage	= swap_writepage,
_
-
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