[PATCH v2] mm/huge_memory: prevent THP_ZERO_PAGE_ALLOC increased twice

[Liu Shixin]

If two or more threads call get_huge_zero_page concurrently,
THP_ZERO_PAGE_ALLOC may increased two or more times. But actually,
this should only count as once since the extra zero pages has been
freed. Redefine its meaning to indicate the times a huge zero page
used for thp is successfully allocated.

Update Documentation/admin-guide/mm/transhuge.rst together.

Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
v1->v2: Update documnet.

 Documentation/admin-guide/mm/transhuge.rst | 7 +++----
 mm/huge_memory.c                           | 2 +-
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index c9c37f16eef8..8e3418ec4503 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -366,10 +366,9 @@ thp_split_pmd
 	page table entry.
 
 thp_zero_page_alloc
-	is incremented every time a huge zero page is
-	successfully allocated. It includes allocations which where
-	dropped due race with other allocation. Note, it doesn't count
-	every map of the huge zero page, only its allocation.
+	is incremented every time a huge zero page used for thp is
+	successfully allocated. Note, it doesn't count every map of
+	the huge zero page, only its allocation.
 
 thp_zero_page_alloc_failed
 	is incremented if kernel fails to allocate
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 88d98241a635..5c83a424803a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -163,7 +163,6 @@ static bool get_huge_zero_page(void)
 		count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);
 		return false;
 	}
-	count_vm_event(THP_ZERO_PAGE_ALLOC);
 	preempt_disable();
 	if (cmpxchg(&huge_zero_page, NULL, zero_page)) {
 		preempt_enable();
@@ -175,6 +174,7 @@ static bool get_huge_zero_page(void)
 	/* We take additional reference here. It will be put back by shrinker */
 	atomic_set(&huge_zero_refcount, 2);
 	preempt_enable();
+	count_vm_event(THP_ZERO_PAGE_ALLOC);
 	return true;
 }
 
-- 
2.25.1