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-rw-r--r--Documentation/vm/page_migration.rst27
1 files changed, 27 insertions, 0 deletions
diff --git a/Documentation/vm/page_migration.rst b/Documentation/vm/page_migration.rst
index 1d6cd7db4e43..68883ac485fa 100644
--- a/Documentation/vm/page_migration.rst
+++ b/Documentation/vm/page_migration.rst
@@ -253,5 +253,32 @@ which are function pointers of struct address_space_operations.
PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag
for own purpose.
+Monitoring Migration
+=====================
+
+The following events (counters) can be used to monitor page migration.
+
+1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a
+ page was migrated. If the page was a non-THP page, then this counter is
+ increased by one. If the page was a THP, then this counter is increased by
+ the number of THP subpages. For example, migration of a single 2MB THP that
+ has 4KB-size base pages (subpages) will cause this counter to increase by
+ 512.
+
+2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
+ _SUCCESS, above: this will be increased by the number of subpages, if it was
+ a THP.
+
+3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.
+
+4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
+
+5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had
+ to be split. After splitting, a migration retry was used for it's sub-pages.
+
+THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or
+PGMIGRATE_FAIL events. For example, a THP migration failure will cause both
+THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.
+
Christoph Lameter, May 8, 2006.
Minchan Kim, Mar 28, 2016.