Re: [PATCH] Btrfs: forced readonly mounts on errors

[liubo <liubo2009@cn.fujitsu.com>]

On 01/18/2011 03:56 AM, Chris Mason wrote:
> Excerpts from liubo's message of 2011-01-06 06:30:25 -0500:
>> This patch comes from "Forced readonly mounts on errors" ideas.
>>
>> As we know, this is the first step in being more fault tolerant of disk
>> corruptions instead of just using BUG() statements.
>>
>> The major content:
>> - add a framework for generating errors that should result in filesystems
>>   going readonly.
>> - keep FS state in disk super block.
>> - make sure that all of resource will be freed and released at umount time.
>> - make sure that after FS is forced readonly on error, there will be no more
>>   disk change before FS is corrected. For this, we should stop write operation.
>>
>> After this patch is applied, the conversion from BUG() to such a framework can
>> happen incrementally.
> 
> I think this is a good overall framework and it will meet our needs
> nicely as we scale up the error handling in the filesystem.
> 
> One concern I have is where we save the error state to disk:
> 
>> +static void __save_error_info(struct btrfs_fs_info *fs_info)
>> +{
>> +    struct btrfs_super_block *disk_super = &fs_info->super_copy;
>> +
>> +    fs_info->fs_state = BTRFS_SUPER_FLAG_ERROR;
>> +    disk_super->flags |= cpu_to_le64(BTRFS_SUPER_FLAG_ERROR);
>> +
>> +    mutex_lock(&fs_info->trans_mutex);
>> +    memcpy(&fs_info->super_for_commit, disk_super,
>> +           sizeof(fs_info->super_for_commit));
>> +    mutex_unlock(&fs_info->trans_mutex);
> 
> The super_for_commit isn't changed until we have a fully consistent set
> of fields in the super block.  The super_copy is changed as the
> transaction progresses.
> 
> So, this memcpy isn't quite safe.  We should simply set the flag on the
> super_for_commit and the super_copy individually.
> 

Got it, thanks for pointing it out.

> I'll make this change and pull it in.  We can build from here.
> 

Great!

thanks,
Liubo

> -chris
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