[PATCH] mm: expland documentation over __read_mostly

[Luis Chamberlain <mcgrof@kernel.org>]

__read_mostly can easily be misused by folks, its not meant for
just read-only data. There are performance reasons for using it, but
we also don't provide any guidance about its use. Provide a bit more
guidance over it use.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---

I sent this 2 years ago, but it fell through the cracks. This time
I'm adding Andrew Morton now, the fix0r-of-falling-through-the-cracks.

Resending as I just saw a patch which doesn't clearly justifiy the
merits of the use of __read_mostly on it.

 include/linux/cache.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/cache.h b/include/linux/cache.h
index 750621e41d1c..8106fb304fa7 100644
--- a/include/linux/cache.h
+++ b/include/linux/cache.h
@@ -15,8 +15,14 @@
 
 /*
  * __read_mostly is used to keep rarely changing variables out of frequently
- * updated cachelines. If an architecture doesn't support it, ignore the
- * hint.
+ * updated cachelines. Its use should be reserved for data that is used
+ * frequently in hot paths. Performance traces can help decide when to use
+ * this. You want __read_mostly data to be tightly packed, so that in the
+ * best case multiple frequently read variables for a hot path will be next
+ * to each other in order to reduce the number of cachelines needed to
+ * execute a critial path. We should be mindful and selective of its use.
+ * ie: if you're going to use it please supply a *good* justification in your
+ * commit log
  */
 #ifndef __read_mostly
 #define __read_mostly
-- 
2.25.1