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

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Li Zefan's message of 2011-06-12 22:20:43 -0400:
> 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).

Right, but that's a lot of trust to put into our small crcs.

-chris