On 05/15/2018 04:35 PM, Duncan wrote:
> Su Yue posted on Tue, 15 May 2018 16:05:01 +0800 as excerpted:
> 
> 
>>
>> On 05/15/2018 03:51 PM, Misono Tomohiro wrote:
>>> Incompat flag of lzo/zstd compression should be set at:
>>>  1. mount time (-o compress/compress-force)
>>>  2. when defrag is done 3. when property is set
>>>
>>> Currently 3. is missing and this commit adds this.
>>>
>>>
>> If I don't misunderstand, compression property of an inode is only apply
>> for *the* inode, not the whole filesystem.
>> So the original logical should be okay.
> 
> But the inode is on the filesystem, and if it's compressed with lzo/zstd, 
> the incompat flag should be set to avoid mounting with an earlier kernel 
> that doesn't understand that compression and would therefore, if we're 
> lucky, simply fail to read the data compressed in that file/inode.  (If 
> we're unlucky it could blow up with kernel memory corruption like James 
> Harvey's current case of unexpected, corrupted compressed data in a nocow 
> file that being nocow, doesn't have csum validation to fail and abort the 
> decompression, and shouldn't be compressed at all.)
> 
> So better to set the incompat flag and refuse to mount at all on kernels 
> that don't have the required compression support.
> 

Get it.
As your conclusion, it's indeed better to set the incompat flag.

Thanks,
Su