mm: hugetlb: proc: check for hugetlb shared PMD in /proc/PID/smaps

Patch series "Fixes for hugetlb mapcount at most 1 for shared PMDs".

This issue of mapcount in hugetlb pages referenced by shared PMDs was
discussed in [1].  The following two patches address user visible behavior
caused by this issue.

[1] https://lore.kernel.org/linux-mm/Y9BF+OCdWnCSilEu@monkey/


This patch (of 2):

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.

[akpm@linux-foundation.org: simplification, per David]
[akpm@linux-foundation.org: hugetlb.h: include page_ref.h for page_count()]
Link: https://lkml.kernel.org/r/20230126222721.222195-2-mike.kravetz@oracle.com
Fixes: 25ee01a ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: mjkravetz

fs/proc/task_mmu.c

@@ -745,9 +745,7 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
 			page = pfn_swap_entry_to_page(swpent);
 	}
 	if (page) {
-		int mapcount = page_mapcount(page);
-
-		if (mapcount >= 2)
+		if (page_mapcount(page) >= 2 || hugetlb_pmd_shared(pte))
 			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
 		else
 			mss->private_hugetlb += huge_page_size(hstate_vma(vma));

include/linux/hugetlb.h

@@ -7,6 +7,7 @@
 #include <linux/fs.h>
 #include <linux/hugetlb_inline.h>
 #include <linux/cgroup.h>
+#include <linux/page_ref.h>
 #include <linux/list.h>
 #include <linux/kref.h>
 #include <linux/pgtable.h>
@@ -1187,6 +1188,18 @@ 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