[PATCH 1/2] mm: hugetlb: proc: check for hugetlb shared PMD in /proc/PID/smaps

[Mike Kravetz]

A hugetlb page will have a mapcount of 1 if mapped by multiple processes
via a shared PMD.  This is because only the first process increases the
map count, and subsequent processes just add the shared PMD page to
their page table.

page_mapcount is being used to decide if a hugetlb page is shared or
private in /proc/PID/smaps.  Pages referenced via a shared PMD were
incorrectly being counted as private.

To fix, check for a shared PMD if mapcount is 1.  If a shared PMD is
found count the hugetlb page as shared.  A new helper to check for a
shared PMD is added.

Fixes: 25ee01a2fca0 ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 fs/proc/task_mmu.c      | 10 ++++++++--
 include/linux/hugetlb.h | 12 ++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e35a0398db63..cb9539879402 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
 
 		if (mapcount >= 2)
 			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
-		else
-			mss->private_hugetlb += huge_page_size(hstate_vma(vma));
+		else {
+			if (hugetlb_pmd_shared(pte))
+				mss->shared_hugetlb +=
+						huge_page_size(hstate_vma(vma));
+			else
+				mss->private_hugetlb +=
+						huge_page_size(hstate_vma(vma));
+		}
 	}
 	return 0;
 }
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index e3aa336df900..8e65920e4363 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1225,6 +1225,18 @@ static inline __init void hugetlb_cma_reserve(int order)
 }
 #endif
 
+#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
+static inline bool hugetlb_pmd_shared(pte_t *pte)
+{
+	return page_count(virt_to_page(pte)) > 1;
+}
+#else
+static inline bool hugetlb_pmd_shared(pte_t *pte)
+{
+	return false;
+}
+#endif
+
 bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr);
 
 #ifndef __HAVE_ARCH_FLUSH_HUGETLB_TLB_RANGE
-- 
2.39.1