[RFC V4] 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>
---
 Documentation/vm/pagemap.txt           |  5 +++++
 fs/proc/page.c                         | 16 +++++++++++++---
 include/linux/huge_mm.h                | 12 ++++++++++++
 include/uapi/linux/kernel-page-flags.h |  1 +
 mm/huge_memory.c                       |  7 +------
 5 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
index 5948e45..fdeb06e 100644
--- a/Documentation/vm/pagemap.txt
+++ b/Documentation/vm/pagemap.txt

@@ -62,6 +62,8 @@ There are three components to pagemap:
     20. NOPAGE
     21. KSM
     22. THP
+    23. BALLOON
+    24. ZERO_PAGE
 
 Short descriptions to the page flags:
 

@@ -102,6 +104,9 @@ Short descriptions to the page flags:
 22. THP
     contiguous pages which construct transparent hugepages
 
+24. ZERO_PAGE
+    zero page for pfn_zero or huge_zero page
+
     [IO related page flags]
  1. ERROR     IO error occurred
  3. UPTODATE  page has up-to-date data