linux-kernel - Re: [PATCH] btrfs: Accept and ignore compression level for lzo

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Message-ID: <810d2b19-47ed-4902-bd8d-eb69bacbf0c6@gmx.com>
Date: Fri, 22 Aug 2025 19:53:59 +0930
From: Qu Wenruo <quwenruo.btrfs@....com>
To: Sun YangKai <sunk67188@...il.com>, calvin@...nvd.org
Cc: clm@...com, dsterba@...e.com, josef@...icpanda.com,
 linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org, neelx@...e.com
Subject: Re: [PATCH] btrfs: Accept and ignore compression level for lzo



在 2025/8/22 19:50, Sun YangKai 写道:
>> The compression level is meaningless for lzo, but before commit
>> 3f093ccb95f30 ("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
>>
>> Restore the old behavior, in case any users were relying on it.
>>
>> Fixes: 3f093ccb95f30 ("btrfs: harden parsing of compression mount options")
>> Signed-off-by: Calvin Owens <calvin@...nvd.org>
>> ---
>>
>>   fs/btrfs/super.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>> index a262b494a89f..7ee35038c7fb 100644
>> --- a/fs/btrfs/super.c
>> +++ b/fs/btrfs/super.c
>> @@ -299,7 +299,7 @@ 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;
>>   		btrfs_set_opt(ctx->mount_opt, COMPRESS);
>>
>> --
>> 2.47.2
> 
> A possible improvement would be to emit a warning in
> btrfs_match_compress_type() when @may_have_level is false but a
> level is still provided. And the warning message can be something like
> "Providing a compression level for {compression_type} is not supported, the
> level is ignored."
> 
> This way:
> 1. users receive a clearer hint about what happened,

I'm fine with the extra warning, but I do not believe those kind of 
users who provides incorrect mount option will really read the dmesg.

> 2. existing setups relying on this behavior continue to work,

Or let them fix the damn incorrect mount option.

I'm fine with warning, but the mount should still fail.
Or those people will never learn to read the doc.

> 3. the @may_have_level semantics remain consistent.
> 
>