Re: [RFC 1/2] ext3: enlarge blocksize and fix rec_len overflow

[Daniel Phillips <phillips@google.com>]

Andreas Dilger wrote:
> On Jun 29, 2006  11:10 -0700, Daniel Phillips wrote:
>>Andreas Dilger wrote:
>>>On Jun 28, 2006  17:50 +0200, Johann Lombardi wrote:
>>>
>>>>ext2/ext3_dir_entry_2 has a 16-bit entry(rec_len) and it would overflow
>>>>with 64KB blocksize.  This patch prevent from overflow by limiting
>>>>rec_len to 65532.
>>>
>>>Having a max rec_len of 65532 is rather unfortunate, since the dir
>>>blocks always need to filled with dir entries.  65536 - 65532 = 4, and
>>>the minimum ext3_dir_entry size is 8 bytes.  I would instead make this
>>>maybe 64 bytes less so that there is room for a filename in the "tail"
>>>dir_entry.
>>
>>Then why not introduce a little symmetry by making max rec_len 2**15 and
>>treat big directory blocks as an array of smaller ones?  I dimly recall
>>the page-cache oriented Ext2 dir code already does this.
> 
> I have no objection to this at all, but I think it will lead to a slightly
> more complex implementation.  We even discussed in the distant past to
> make large directories a series of 4kB "chunks", for fs blocksize >= 4kB.
> This has negative implications for large filenames because the internal
> free space fragmentation is high, but has the advantage that it might
> eventually still be usable if we can get blocksize > PAGE_SIZE.
> 
> The difficulty is that when freeing dir entires you would have to be
> concerned with a merging a dir_entry that is spanning the middle
> of a 2^16 block.

That is easy, just don't let an entry span subblocks by not letting
delete merge past the end of a subblock, just a minor tweak.  New block
initialization needs an outer loop on subblocks and that's it, I think.

Regards,

Daniel