Re: [PATCH 2/5] btrfs: correct a message on setting nodatacow

[Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>]

Hi Qu,

Thank you for your comment.

(2014/09/19 11:03), Qu Wenruo wrote:
>
> -------- Original Message --------
> Subject: [PATCH 2/5] btrfs: correct a message on setting nodatacow
> From: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
> To: linux-btrfs@vger.kernel.org <linux-btrfs@vger.kernel.org>
> Date: 2014年09月18日 16:28
>> From: Naohiro Aota <naota@elisp.net>
>>
>> If we set nodatacow mount option after compress-force option,
>> we don't get compression disabling message.
>>
>> ===
>> $ sudo mount -o remount,compress-force,nodatacow /; dmesg|tail -n 3
>> [ 3845.719047] BTRFS info (device vda2): force zlib compression
>> [ 3845.719052] BTRFS info (device vda2): setting nodatacow
>> [ 3845.719055] BTRFS info (device vda2): disk space caching is enabled
>> ===
>>
>> Signed-off-by: Naohiro Aota <naota@elisp.net>
>> Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
>> ---
>>   fs/btrfs/super.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>> index d1c5b6d..d131098 100644
>> --- a/fs/btrfs/super.c
>> +++ b/fs/btrfs/super.c
>> @@ -462,8 +462,7 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
>>               break;
>>           case Opt_nodatacow:
>>               if (!btrfs_test_opt(root, NODATACOW)) {
>> -                if (!btrfs_test_opt(root, COMPRESS) ||
>> -                    !btrfs_test_opt(root, FORCE_COMPRESS)) {
>> +                if (btrfs_test_opt(root, COMPRESS)) {
>>                       btrfs_info(root->fs_info,
>>                              "setting nodatacow, compression disabled");
>>                   } else {
>> -- 1.8.3.1
>>
>> --
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> Although the patch makes the output ok, the core problem is missing conflict options check.
>
> compress-force mount options implies datacow and datasum, but following nodatasum will disable datasum and compress, in fact they are conflicting mount option...
>
> Even the current behavior(later mount option will override previous ones) provides great tolerance,
> IMO there should better be some conflicting check for mount options.
>
> For example, we first save all the mount options passed in into a temporary bitmaps to finds out the conflicting
> and only when they contains no conflicts, set the mount options to fs_info.
> (Maybe bitmap is not enough for this case, since we can't distinguish default value and value to be set?)
>
> What do you think about this idea ?

I'm against your idea for two reasons and it's better to
stay in current behavior though it's a bit complex.

First, the rule "last one wins" is not only a conventional rule,
but also is what mount(8) says.

https://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/sys-utils/mount.8#n253

======
The usual behavior is that the last option wins if there are conflicting
ones.
======

Second, if we change the behavior, we would break existing
systems. At worst case, users would fail to boot their system
after updating kernel, because of the failure of mounting
Btrfs at the init process.

Thanks,
Satoru

>
> Thanks,
> Qu
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