From 7781d1a2daa6564c3988dc268eb0680e057027f5 Mon Sep 17 00:00:00 2001
From: Li Zhang <zhanglikernel@gmail.com>
Date: Sun, 16 Jan 2022 16:52:43 +0800
Subject: [PATCH] btrfs-progs: props: don't translate value of compression=none
References: JSC#PED-1711

Currently, if user specifies value 'no' or 'none' on the command line,
it gets translated to an empty value that is passed to kernel. There was
a change in kernel 5.14 done by commit 5548c8c6f55b ("btrfs: props:
change how empty value is interpreted") that changes the behaviour
in that case.

The empty value is supposed to mean 'the default value' for any
property. For compression there is a need to distinguish resetting the
value and also setting the NOCOMPRESS property. The translation to empty
value makes that impossible.

The explanation and behaviour copied from the kernel patch:

    Old behaviour:

      $ lsattr file
      ---------------------- file
      # the NOCOMPRESS bit is set
      $ btrfs prop set file compression ''
      $ lsattr file
      ---------------------m file

    This is equivalent to 'btrfs prop set file compression no' in current
    btrfs-progs as the 'no' or 'none' values are translated to an empty
    string.

    This is where the new behaviour is different: empty string drops the
    compression flag (-c) and nocompress (-m):

      $ lsattr file
      ---------------------- file
      # No change
      $ btrfs prop set file compression ''
      $ lsattr file
      ---------------------- file
      $ btrfs prop set file compression lzo
      $ lsattr file
      --------c------------- file
      $ btrfs prop get file compression
      compression=lzo
      $ btrfs prop set file compression ''
      # Reset to the initial state
      $ lsattr file
      ---------------------- file
      # Set NOCOMPRESS bit
      $ btrfs prop set file compression no
      $ lsattr file
      ---------------------m file

    This obviously brings problems with backward compatibility, so this
    patch should not be backported without making sure the updated
    btrfs-progs are also used and that scripts have been updated to use the
    new semantics.

    Summary:

    - old kernel:
      no, none, "" - set NOCOMPRESS bit
    - new kernel:
      no, none - set NOCOMPRESS bit
      "" - drop all compression flags, ie. COMPRESS and NOCOMPRESS

Signed-off-by: Li Zhang <zhanglikernel@gmail.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

---
 Documentation/btrfs-property.8        |   11 +++++++++--
 Documentation/btrfs-property.asciidoc |    3 ++-
 cmds/property.c                       |    2 --
 3 files changed, 11 insertions(+), 5 deletions(-)

--- a/Documentation/btrfs-property.8
+++ b/Documentation/btrfs-property.8
@@ -82,10 +82,17 @@
 compression algorithm set for an inode, possible values:
 \fIlzo\fR,
 \fIzlib\fR,
-\fIzstd\fR\&. To disable compression use "" (empty string),
+\fIzstd\fR\&. To disable compression use
 \fIno\fR
 or
-\fInone\fR\&.
+\fInone\fR\&. Empty value resets the property and sets a default value.
+.br
+.PP
+.ps +1
+\fBNote\fR
+.ps -1
+.br
+An empty value resetting compression property has changed since kernel 5.14. Earlier versions would disable compression.
 .RE
 .RE
 .PP
--- a/Documentation/btrfs-property.asciidoc
+++ b/Documentation/btrfs-property.asciidoc
@@ -47,7 +47,8 @@
 device as object. For a mounted filesystem, specify a mount point.
 compression::::
 compression algorithm set for an inode, possible values: 'lzo', 'zlib', 'zstd'.
-To disable compression use "" (empty string), 'no' or 'none'.
+To disable compression use 'no' or 'none'. An empty string "" resets the property and sets a default value.
+'Note:' An empty value resetting compression property has changed since kernel 5.14. Earlier versions would disable compression.
 
 *list* [-t <type>] <object>::
 Lists available properties with their descriptions for the given object.
--- a/cmds/property.c
+++ b/cmds/property.c
@@ -126,8 +126,6 @@
 	xattr_name[XATTR_BTRFS_PREFIX_LEN + strlen(name)] = '\0';
 
 	if (value) {
-		if (strcmp(value, "no") == 0 || strcmp(value, "none") == 0)
-			value = "";
 		sret = fsetxattr(fd, xattr_name, value, strlen(value), 0);
 	} else {
 		sret = fgetxattr(fd, xattr_name, NULL, 0);