[RFC V2] mm:add KPF_ZERO_PAGE flag for /proc/kpageflags

This patch add KPF_ZERO_PAGE flag for zero_page,
so that userspace process can notice zero_page from
/proc/kpageflags, and then do memory analysis more accurately.

Signed-off-by: Yalin Wang <redacted>
---
 fs/proc/page.c                         | 14 +++++++++++---
 include/linux/huge_mm.h                | 12 ++++++++++++
 include/uapi/linux/kernel-page-flags.h |  1 +
 mm/huge_memory.c                       |  7 +------
 4 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 1e3187d..dbe5630 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c

@@ -5,6 +5,7 @@
 #include <linux/ksm.h>
 #include <linux/mm.h>
 #include <linux/mmzone.h>
+#include <linux/huge_mm.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
 #include <linux/hugetlb.h>

@@ -121,9 +122,16 @@ u64 stable_page_flags(struct page *page)
 	 * just checks PG_head/PG_tail, so we need to check PageLRU/PageAnon
 	 * to make sure a given page is a thp, not a non-huge compound page.
 	 */
-	else if (PageTransCompound(page) && (PageLRU(compound_head(page)) ||
-					     PageAnon(compound_head(page))))
-		u |= 1 << KPF_THP;
+	else if (PageTransCompound(page)) {
+		struct page *head = compound_head(page);
+
+		if (PageLRU(head) || PageAnon(head))
+			u |= 1 << KPF_THP;
+		else if (is_huge_zero_page(head))
+			u |= 1 << KPF_ZERO_PAGE;