Re: [PATCH] Btrfs: use the normal checksumming infrastructure for free space cache

[Li Zefan <lizf@cn.fujitsu.com>]

Chris Mason wrote:
> Excerpts from Li Zefan's message of 2011-06-12 21:52:32 -0400:
>> Josef Bacik wrote:
>>> We used to store the checksums of the space cache directly in the space cache,
>>> however that doesn't work out too well if we have more space than we can fit the
>>> checksums into the first page.  So instead use the normal checksumming
>>> infrastructure.  There were problems with doing this originally but those
>>> problems don't exist now so this works out fine.  Thanks,
>>>
>>
>> This looks great, so I'll drop my patch that extends the original code to
>> allow more than 1 crc page.
> 
> I do like them a lot, but what happens when a special case crc kernel
> mounts a free space cache created by this patch?
> 

The generation number will be checked, and then we'll get this warning:

"btrfs: space cache generation xx does not match inode yy"

If xx happens to be equal to yy, crcs will be checked and this warning
will pop up:

"btrfs: crc mismatch for page zz"

but it won't crash (not tested).