mempolicy: clarify what zone reclaim means

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.

[joshua.hahnjy@gmail.com: remove the reference to the vm.zone_reclaim_mode sysctl as an ABI]
  Link: https://lkml.kernel.org/r/20250806134404.2000234-1-joshua.hahnjy@gmail.com
Link: https://lkml.kernel.org/r/20250805205048.1518453-1-joshua.hahnjy@gmail.com
Link: https://lore.kernel.org/linux-mm/1579005573-58923-1-git-send-email-alex.shi@linux.alibaba.com/ [1]
Link: https://lore.kernel.org/linux-mm/20200626003459.D8E015CA@viggo.jf.intel.com/ [2]
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: SeongJae Park <sj@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Byungchul Park <byungchul@sk.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
index 1f9bb10d1a47..8fbbe613611a 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 */
 
 /*
- * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
- * ABI.  New bits are OK, but existing bits can never change.
+ * 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.
+ * 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 */