[PATCH] mm, hwpoison: Add comment describing when to add new cases

[Andi Kleen <andi@firstfloor.org>]

From: Andi Kleen <ak@linux.intel.com>

Here's another comment fix for hwpoison.

It describes the "guiding principle" on when to add new
memory error recovery code.

v2: Add URL
Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 mm/memory-failure.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 25c2054..97e44d3 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -20,6 +20,14 @@
  * this code has to be extremely careful. Generally it tries to use 
  * normal locking rules, as in get the standard locks, even if that means 
  * the error handling takes potentially a long time.
+ *
+ * It can be very tempting to add handling for obscure cases here.
+ * In general any code for handling new cases should only be added iff:
+ * - You know how to test it.
+ * - You have a test that can be added to mce-test
+ *   https://git.kernel.org/cgit/utils/cpu/mce/mce-test.git/
+ * - The case actually shows up as a frequent (top 10) page state in
+ *   tools/vm/page-types when running a real workload.
  * 
  * There are several operations here with exponential complexity because
  * of unsuitable VM data structures. For example the operation to map back 
-- 
1.9.3

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