btrfs: respect the max size in the header when activating swap file

If we extended the size of a swapfile after its header was created (by the
mkswap utility) and then try to activate it, we will map the entire file
when activating the swap file, instead of limiting to the max size defined
in the swap file's header.

Currently test case generic/643 from fstests fails because we do not
respect that size limit defined in the swap file's header.

So fix this by not mapping file ranges beyond the max size defined in the
swap header.

This is the same type of bug that iomap used to have, and was fixed in
commit 36ca7943ac ("mm/swap: consider max pages in
iomap_swapfile_add_extent").

Fixes: ed46ff3d42 ("Btrfs: support swap files")
CC: stable@vger.kernel.org # 5.4+
Reviewed-and-tested-by: Josef Bacik <josef@toxicpanda.com
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -10269,9 +10269,19 @@ static int btrfs_add_swap_extent(struct swap_info_struct *sis,
 				 struct btrfs_swap_info *bsi)
 {
 	unsigned long nr_pages;
+	unsigned long max_pages;
 	u64 first_ppage, first_ppage_reported, next_ppage;
 	int ret;
 
+	/*
+	 * Our swapfile may have had its size extended after the swap header was
+	 * written. In that case activating the swapfile should not go beyond
+	 * the max size set in the swap header.
+	 */
+	if (bsi->nr_pages >= sis->max)
+		return 0;
+
+	max_pages = sis->max - bsi->nr_pages;
 	first_ppage = ALIGN(bsi->block_start, PAGE_SIZE) >> PAGE_SHIFT;
 	next_ppage = ALIGN_DOWN(bsi->block_start + bsi->block_len,
 				PAGE_SIZE) >> PAGE_SHIFT;
@@ -10279,6 +10289,7 @@ static int btrfs_add_swap_extent(struct swap_info_struct *sis,
 	if (first_ppage >= next_ppage)
 		return 0;
 	nr_pages = next_ppage - first_ppage;
+	nr_pages = min(nr_pages, max_pages);
 
 	first_ppage_reported = first_ppage;
 	if (bsi->start == 0)