util-linux/util-linux-swapon-btrfs-limitations

From: Jeff Mahoney <jeffm@suse.com>
Subject: swapon: Document btrfs limitation with swapfiles
References: bnc#616617

 Btrfs, as of 2.6.35, is unable to allow swapfiles to be used on its
 filesystems. This is due to the swapfile implementation wanting to build
 an extent map of each block in the file and expecting it to be static
 for the life of the swapfile.

 Btrfs can't guarantee this and refuses to return the mapping. The swapfile
 implementation just makes a comment about there being holes in the file -
 but that's how btrfs denies the mapping.

 This patch adds a section to the swapon manpage to document it.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>

---
 mount/swapon.8 |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/mount/swapon.8
+++ b/mount/swapon.8
@@ -167,6 +167,18 @@ automatically detects and rewrites swap
 suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
 do it, then we get data corruption the next time an attempt at unsuspending is
 made.
+.PP
+.B swapon
+may not work correctly when using a swap file with some versions of btrfs.
+This is due to the swap file implementation in the kernel expecting to be able
+to write to the file directly, without the assistance of the file system.
+Since btrfs is a copy-on-write file system, the file location may not be
+static and corruption can result. Btrfs actively disallows the use of files
+on its file systems by refusing to map the file. This can be seen in the system
+log as "swapon: swapfile has holes." One possible workaround is to map the
+file to a loopback device. This will allow the file system to determine the
+mapping properly but may come with a performance impact.
+
 .SH SEE ALSO
 .BR swapon (2),
 .BR swapoff (2),