[PATCH] docs/power: note Btrfs limitation with FIBMAP for swap file offset

[Arun Rao Balappa <arunraobalappa@gmail.com>]

On Btrfs, the FIBMAP ioctl does not return physical block addresses.
Tools such as filefrag therefore cannot determine the correct swap file
offset for use as resume_offset. Document the correct btrfs-progs
command to use instead.

Signed-off-by: Arun Rao Balappa <arunraobalappa@gmail.com>
---
 Documentation/power/swsusp-and-swap-files.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Documentation/power/swsusp-and-swap-files.rst b/Documentation/power/swsusp-and-swap-files.rst
index a33a2919d..44ffe02b3 100644
--- a/Documentation/power/swsusp-and-swap-files.rst
+++ b/Documentation/power/swsusp-and-swap-files.rst
@@ -35,6 +35,14 @@ FIBMAP ioctl and determine the location of the file's swap header, as the
 offset, in <PAGE_SIZE> units, from the beginning of the partition which
 holds the swap file.
 
+.. note::
+
+   On Btrfs, the FIBMAP ioctl does not return physical block addresses, so
+   tools such as ``filefrag`` cannot be used to determine the correct offset.
+   On Btrfs, use btrfs-progs instead::
+
+      btrfs inspect-internal map-swapfile -r <swap_file_path>
+
 3) Add the following parameters to the kernel command line::
 
     resume=<swap_file_partition> resume_offset=<swap_file_offset>
-- 
2.53.0