On Tue, Jul 27, 2021 at 7:18 AM Mike Kravetz [off-list ref] wrote:

On 7/14/21 2:17 AM, Muchun Song wrote:

quoted

Introduce save_page_flags to return the page flags which can cooperate
with show_page_flags. If we want to hihe some page flags from users, it
will be useful to alter save_page_flags directly. This is a preparation
for the next patch to hide some page flags from users.

Signed-off-by: Muchun Song <redacted>
---
 include/trace/events/mmflags.h  | 3 +++
 include/trace/events/page_ref.h | 8 ++++----
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 390270e00a1d..69cb84b1257e 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h

@@ -121,6 +121,9 @@ IF_HAVE_PG_IDLE(PG_idle,          "idle"          )               \
 IF_HAVE_PG_ARCH_2(PG_arch_2,         "arch_2"        )               \
 IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")

+#define save_page_flags(page)                                                \
+     (((page)->flags & ~PAGEFLAGS_MASK))
+

Looking ahead to the next patch, this is changed to hide the PG_head
flag for 'fake' head pages.

IIRC, all vmemmap pages except the first will be mapped read only.  So,
all vmemmap pages with 'fake' head pages will be read only.

It seems that all the modified trace events below are associated with
updates to page structs.  Therefore, it seems these events will never
experience a 'fake' head page.  Am I missing something?

Totally right. I didn't realize this point before. We cannot see a fake page
struct in page refcount tracing. So this patch is definitely pointless. I'll
drop it in the next version. Thanks Mike.

--
Mike Kravetz