mm/huge_memory: prevent THP_ZERO_PAGE_ALLOC increased twice

A user who reads THP_ZERO_PAGE_ALLOC may be more concerned about the huge
zero pages that are really allocated for thp.  It is misleading to
increase THP_ZERO_PAGE_ALLOC twice if two threads call get_huge_zero_page
concurrently.  Don't increase the value if the huge page is not really
used.

Update Documentation/admin-guide/mm/transhuge.rst to suit.

Link: https://lkml.kernel.org/r/20220909021653.3371879-1-liushixin2@huawei.comSigned-off-by: Liu Shixin <liushixin2@huawei.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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

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;
 }