Re: [PATCH] ext4: Fix to read empty directory blocks correctly in 64k blocksize filesystems

[Andreas Dilger <adilger@sun.com>]

On Feb 11, 2009  10:15 -0500, Theodore Ts'o wrote:
> On Wed, Feb 11, 2009 at 12:48:16AM -0500, Andreas Dilger wrote:
> > I'm glad that the MAX_REC_LEN value is being kept, because "0" is
> > too easily hit due to disk corruption.
> 
> I'm not too worried about that, actually.  If there is a disk
> corruption, we will detect it easily enough no matter which encoding
> we use.  I am thinking though that neither 65535 nor 0 is the best way
> of encoding 65536.  In fact, I would suggest the encoding 0x01.
> Specifically, I suggest:
> 
> 	     (rec_len & 65532) | ((rec_len >> 16) & 3)
> 
> This encodes valid rec_len values in the range 0 through 2**18-4, and
> allows for maximum block sizes of up to 256k.  To retain backwards
> compatibility, and to allow for a 256k blocksize, we can have a
> special case where a rec_len of either 0 or 65535 means
> EXT4_BLOCK_SIZE(s).
> 
> It's unlikely we'll see VM pages of up to 256k, but at some point we
> might find that the Linux VM has been enhanced to support filesystem
> block sizes > than the VM page size, at which point it might be useful
> for some applications to allow very large filesystem block sizes.

There are probably a dozen other places in the ext* code that expect
blocksize <= 65536, so I don't think this new encoding is really helping
us at all.  We are already restricted to 2^32-1 inodes due to the dirent
format and I expect we will have changed the dirent format by that time
anyways.

I'd rather keep this change as simple as possible (i.e. the original
65535 or 0 values, preferring 65535).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.