[PATCH 2/2] iomap: allow NULL swap info bdev when activating swapfile

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

All current users of the iomap swapfile activation mechanism are block
device filesystems.  This means that claim_swapfile will set
swap_info_struct::bdev to inode->i_sb->s_bdev of the swap file.

However, a subsequent patch to fuse will add iomap infrastructure so
that fuse servers can be asked to provide file mappings specifically for
swap files.  The fuse server isn't required to set s_bdev (by mounting
as fuseblk) so s_bdev might be null.  For this case, we want to set
sis::bdev from the first mapping.

To make this work robustly, we must explicitly check that each mapping
provides a bdev and that there's no way we can succeed at collecting
swapfile pages without a block device.

And just to be clear: fuse-iomap servers will have to respond to an
explicit request for swapfile activation.  It's not like fuseblk, where
responding to bmap means swapfiles work even if that wasn't expected.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/iomap/swapfile.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)


diff --git a/fs/iomap/swapfile.c b/fs/iomap/swapfile.c
index 0db77c449467a7..9d9f4e84437df5 100644
--- a/fs/iomap/swapfile.c
+++ b/fs/iomap/swapfile.c
@@ -112,6 +112,13 @@ static int iomap_swapfile_iter(struct iomap_iter *iter,
 	if (iomap->flags & IOMAP_F_SHARED)
 		return iomap_swapfile_fail(isi, "has shared extents");
 
+	/* Swapfiles must be backed by a block device */
+	if (!iomap->bdev)
+		return iomap_swapfile_fail(isi, "is not on a block device");
+
+	if (iter->pos == 0 && !isi->sis->bdev)
+		isi->sis->bdev = iomap->bdev;
+
 	/* Only one bdev per swap file. */
 	if (iomap->bdev != isi->sis->bdev)
 		return iomap_swapfile_fail(isi, "outside the main device");
@@ -184,6 +191,16 @@ int iomap_swapfile_activate(struct swap_info_struct *sis,
 		return -EINVAL;
 	}
 
+	/*
+	 * If this swapfile doesn't have a block device, reject this useless
+	 * swapfile to prevent confusion later on.
+	 */
+	if (sis->bdev == NULL) {
+		pr_warn(
+ "swapon: No block device for swap file but usage pages?!\n");
+		return -EINVAL;
+	}
+
 	*pagespan = 1 + isi.highest_ppage - isi.lowest_ppage;
 	sis->max = isi.nr_pages;
 	sis->pages = isi.nr_pages - 1;