btrfs: accept and ignore compression level for lzo

The compression level is meaningless for lzo, but before commit
3f093cc ("btrfs: harden parsing of compression mount options"),
it was silently ignored if passed.

After that commit, passing a level with lzo fails to mount:

    BTRFS error: unrecognized compression value lzo:1

It seems reasonable for users to expect that lzo would permit a numeric
level option, as all the other algos do, even though the kernel's
implementation of LZO currently only supports a single level. Because it
has always worked to pass a level, it seems likely to me that users in
the real world are relying on doing so.

This patch restores the old behavior, giving "lzo:N" the same semantics
as all of the other compression algos.

To be clear, silly variants like "lzo:one", "lzo:the_first_option", or
"lzo:armageddon" also used to work. This isn't meant to suggest that
any possible mis-interpretation of mount options that once worked must
continue to work forever. This is an exceptional case where it makes
sense to preserve compatibility, both because the mis-interpretation is
reasonable, and because nothing tangible is sacrificed.

Fixes: 3f093cc ("btrfs: harden parsing of compression mount options")
Reviewed-by: Daniel Vacek <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Calvin Owens <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: jcalvinowens

fs/btrfs/super.c

@@ -297,9 +297,12 @@ static int btrfs_parse_compress(struct btrfs_fs_context *ctx,
 		btrfs_set_opt(ctx->mount_opt, COMPRESS);
 		btrfs_clear_opt(ctx->mount_opt, NODATACOW);
 		btrfs_clear_opt(ctx->mount_opt, NODATASUM);
-	} else if (btrfs_match_compress_type(string, "lzo", false)) {
+	} else if (btrfs_match_compress_type(string, "lzo", true)) {
 		ctx->compress_type = BTRFS_COMPRESS_LZO;
-		ctx->compress_level = 0;
+		ctx->compress_level = btrfs_compress_str2level(BTRFS_COMPRESS_LZO,
+							       string + 3);
+		if (string[3] == ':' && string[4])
+			btrfs_warn(NULL, "Compression level ignored for LZO");
 		btrfs_set_opt(ctx->mount_opt, COMPRESS);
 		btrfs_clear_opt(ctx->mount_opt, NODATACOW);
 		btrfs_clear_opt(ctx->mount_opt, NODATASUM);