Re: [PATCH] btrfs-progs: reserve < 0 return value for errnos

[Anand Jain <anand.jain@oracle.com>]

On 10/27/2017 04:04 PM, Qu Wenruo wrote:
> 
> 
> On 2017年10月27日 15:54, Nikolay Borisov wrote:
>>
>>
>> On 27.10.2017 10:31, Anand Jain wrote:
>>> btrfs_read_dev_super() returns -1 upon not finding a suitable SB,
>>> change that to return 1 instead, so that it can reserve the < 0
>>> values for the errno communications.
>>>
>>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>>
>> I believe this is buggy since it will be masking errors. We call
>> btrfs_read_dev_super in open_ctree_with_broken_chunk if the function
>> returns 1 we eventually exit as: return ERR_PTR(ret); And then perform
>> IS_ERR on the returned value. IS_ERR is essentially:
>> ((x) >= (unsigned long)-MAX_ERRNO) where x could be 1 and MAX_ERRNO is 4095
>>
>> So you just broke all functions which return the ret of
>> btrfs_read_dev_super as ERR_PTR.
>>
>>
>> NAK
> 
> This doesn't looks good to me either.
> 
> No suitable superblock is already an error.
> Why not just returning -ENOENT and let caller who really needs to
> distinguish this to catch that -ENOENT?

  Thanks for pointing out. Will fix it. IMO, -ENOENT is good for now.

  (Though the actual cleanup would be to..
     There are two parts in btrfs_read_dev_super() one to read
     only the BTRFS_SUPER_INFO_OFFSET SB. And the other to pick
     the latest SB by generation #. So splitting them two separate
     functions will let us know where do we use what in a much
     clearer way. And for 2nd pread64 we should probably use errno.)

Thanks, Anand


> Thanks,
> Qu
> 
>>
>>> ---
>>> An independent patch, not related to any recent patch sent to ML.
>>>
>>>   disk-io.c | 2 +-
>>>   utils.c   | 2 +-
>>>   volumes.c | 2 +-
>>>   3 files changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/disk-io.c b/disk-io.c
>>> index f5edc4796619..ba87bb4ce6da 100644
>>> --- a/disk-io.c
>>> +++ b/disk-io.c
>>> @@ -1491,7 +1491,7 @@ int btrfs_read_dev_super(int fd, struct btrfs_super_block *sb, u64 sb_bytenr,
>>>   		}
>>>   	}
>>>   
>>> -	return transid > 0 ? 0 : -1;
>>> +	return transid > 0 ? 0 : 1;
>>>   }
>>>   
>>>   static int write_dev_supers(struct btrfs_fs_info *fs_info,
>>> diff --git a/utils.c b/utils.c
>>> index 524f463d3140..7574cda8151c 100644
>>> --- a/utils.c
>>> +++ b/utils.c
>>> @@ -1678,7 +1678,7 @@ int get_fs_info(const char *path, struct btrfs_ioctl_fs_info_args *fi_args,
>>>   		disk_super = (struct btrfs_super_block *)buf;
>>>   		ret = btrfs_read_dev_super(fd, disk_super,
>>>   					   BTRFS_SUPER_INFO_OFFSET, 0);
>>> -		if (ret < 0) {
>>> +		if (ret) {
>>>   			ret = -EIO;
>>>   			goto out;
>>>   		}
>>> diff --git a/volumes.c b/volumes.c
>>> index 2209e5a9100b..87008bfdd586 100644
>>> --- a/volumes.c
>>> +++ b/volumes.c
>>> @@ -268,7 +268,7 @@ int btrfs_scan_one_device(int fd, const char *path,
>>>   
>>>   	disk_super = (struct btrfs_super_block *)buf;
>>>   	ret = btrfs_read_dev_super(fd, disk_super, super_offset, sbflags);
>>> -	if (ret < 0)
>>> +	if (ret)
>>>   		return -EIO;
>>>   	devid = btrfs_stack_device_id(&disk_super->dev_item);
>>>   	if (btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_METADUMP)
>>>
>> --
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>