[PATCH 17/21] userfaultfd: remap_pages: swp_entry_swapcount() preparation

[Andrea Arcangeli <aarcange@redhat.com>]

Provide a new swapfile method for remap_pages() to verify the swap
entry is mapped only in one vma before relocating the swap entry in a
different virtual address. Otherwise if the swap entry is mapped in
multiple vmas, when the page is swapped back in, it could get mapped
in a non linear way in some anon_vma.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 include/linux/swap.h |  6 ++++++
 mm/swapfile.c        | 13 +++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4759491..9adda11 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -436,6 +436,7 @@ extern unsigned int count_swap_pages(int, int);
 extern sector_t map_swap_page(struct page *, struct block_device **);
 extern sector_t swapdev_block(int, pgoff_t);
 extern int page_swapcount(struct page *);
+extern int swp_entry_swapcount(swp_entry_t entry);
 extern struct swap_info_struct *page_swap_info(struct page *);
 extern int reuse_swap_page(struct page *);
 extern int try_to_free_swap(struct page *);
@@ -527,6 +528,11 @@ static inline int page_swapcount(struct page *page)
 	return 0;
 }
 
+static inline int swp_entry_swapcount(swp_entry_t entry)
+{
+	return 0;
+}
+
 #define reuse_swap_page(page)	(page_mapcount(page) == 1)
 
 static inline int try_to_free_swap(struct page *page)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 63f55cc..04c7621 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -874,6 +874,19 @@ int page_swapcount(struct page *page)
 	return count;
 }
 
+int swp_entry_swapcount(swp_entry_t entry)
+{
+	int count = 0;
+	struct swap_info_struct *p;
+
+	p = swap_info_get(entry);
+	if (p) {
+		count = swap_count(p->swap_map[swp_offset(entry)]);
+		spin_unlock(&p->lock);
+	}
+	return count;
+}
+
 /*
  * We can write to an anon page without COW if there are no other references
  * to it.  And as a side-effect, free up its swap: because the old content