[PATCH] swap: fix swapfile read/write offset

[Jens Axboe <axboe@xxxxxxxxx>]

We're not factoring in the start of the file for where to write and
read the swapfile, which leads to very unfortunate side effects of
writing where we should not be...

Fixes: 48d15436fde6 ("mm: remove get_swap_bio")
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

---

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 32f665b1ee85..4cc6ec3bf0ab 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -485,6 +485,7 @@ struct backing_dev_info;
 extern int init_swap_address_space(unsigned int type, unsigned long nr_pages);
 extern void exit_swap_address_space(unsigned int type);
 extern struct swap_info_struct *get_swap_device(swp_entry_t entry);
+sector_t swap_page_sector(struct page *page);
 
 static inline void put_swap_device(struct swap_info_struct *si)
 {
diff --git a/mm/page_io.c b/mm/page_io.c
index 485fa5cca4a2..c493ce9ebcf5 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -254,11 +254,6 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
 	return ret;
 }
 
-static sector_t swap_page_sector(struct page *page)
-{
-	return (sector_t)__page_file_index(page) << (PAGE_SHIFT - 9);
-}
-
 static inline void count_swpout_vm_event(struct page *page)
 {
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
diff --git a/mm/swapfile.c b/mm/swapfile.c
index f039745989d2..7a8636c6c9ff 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -199,8 +199,8 @@ static int discard_swap(struct swap_info_struct *si)
 	return err;		/* That will often be -EOPNOTSUPP */
 }
 
-static struct swap_extent *
-offset_to_swap_extent(struct swap_info_struct *sis, unsigned long offset)
+struct swap_extent *offset_to_swap_extent(struct swap_info_struct *sis,
+					  unsigned long offset)
 {
 	struct swap_extent *se;
 	struct rb_node *rb;
@@ -1858,6 +1858,19 @@ sector_t swapdev_block(int type, pgoff_t offset)
 	return se->start_block + (offset - se->start_page);
 }
 
+sector_t swap_page_sector(struct page *page)
+{
+	struct swap_info_struct *sis = page_swap_info(page);
+	struct swap_extent *se;
+	sector_t sector;
+	pgoff_t offset;
+
+	offset = __page_file_index(page);
+	se = offset_to_swap_extent(sis, offset);
+	sector = se->start_block + (offset - se->start_page);
+	return sector << (PAGE_SHIFT - 9);
+}
+
 /*
  * Return either the total number of swap pages of given type, or the number
  * of free pages of that type (depending on @free)

-- 
Jens Axboe