[PATCH v3] mempolicy: Clarify what zone reclaim means

[Joshua Hahn <joshua.hahnjy@gmail.com>]

The zone_reclaim_mode API controls the reclaim behavior when a node runs out of
memory. Contrary to its user-facing name, it is internally referred to as
"node_reclaim_mode".

This can be confusing. But because we cannot change the name of the API since
it has been in place since at least 2.6, let's try to be more explicit about
what the behavior of this API is. 

Change the description to clarify what zone reclaim entails, and be explicit
about the RECLAIM_ZONE bit, whose purpose has led to some confusion in the
past already [1] [2].

While at it, also soften the warning about changing these bits.

[1] https://lore.kernel.org/linux-mm/1579005573-58923-1-git-send-email-alex.shi@linux.alibaba.com/
[2] https://lore.kernel.org/linux-mm/20200626003459.D8E015CA@viggo.jf.intel.com/

Acked-by: SeongJae Park <sj@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
v2 --> v3:
- Fixed typos
- Softend wording from "never" --> "should not"

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

diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
index 1f9bb10d1a47..683c130782f0 100644
--- a/include/uapi/linux/mempolicy.h
+++ b/include/uapi/linux/mempolicy.h
@@ -66,10 +66,16 @@ enum {
 #define MPOL_F_MORON	(1 << 4) /* Migrate On protnone Reference On Node */
 
 /*
+ * Enabling zone reclaim means the page allocator will attempt to fulfill
+ * the allocation request on the current node by triggering reclaim and
+ * trying to shrink the current node.
+ * Fallback allocations on the next candidates in the zonelist are considered
+ * when reclaim fails to free up enough memory in the current node/zone.
+ *
  * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
- * ABI.  New bits are OK, but existing bits can never change.
+ * ABI.  New bits are OK, but existing bits should not be changed.
  */
-#define RECLAIM_ZONE	(1<<0)	/* Run shrink_inactive_list on the zone */
+#define RECLAIM_ZONE	(1<<0)	/* Enable zone reclaim */
 #define RECLAIM_WRITE	(1<<1)	/* Writeout pages during reclaim */
 #define RECLAIM_UNMAP	(1<<2)	/* Unmap pages during reclaim */
 

base-commit: 6bcdbd62bd56e6d7383f9e06d9d148935b3c9b73
-- 
2.47.3