Re: [PATCH 04/10] AXFS: axfs_inode.c

Jared Hulbert
2008-08-22T03:23:57Z

> Squashfs has much larger block sizes than cramfs (last time I looked it was
> limited to 4K blocks), and it compresses the metadata which helps to get
> better compression. But tail merging (fragments in Squashfs terminology) is
> obviously a major reason why Squashfs gets good compression.
>
> The AXFS code is rather obscure but it doesn't look to me that it does tail
> merging. The following code wouldn't work if the block in question was a
> tail contained in a larger block. It assumes the block extends to the end
> of the compressed block (cblk_size - cnode_offset).

A c_block is the unit that gets compressed. It can contain multiple
c_nodes. The c_block can be PAGE_SIZE to 4GB in size, in theory :)
The c_nodes can be 1B to PAGE_SIZE. in any alignment. I pack many
tails as c_nodes in a c_block.

>>> + max_len = cblk_size - cnode_offset;
>>> + len = max_len > PAGE_CACHE_SIZE ? PAGE_CACHE_SIZE :
>>> max_len;
>>> + src = (void *)((unsigned long)cblk0 + cnode_offset);
>>> + memcpy(pgdata, src, len);
>
> Perhaps the AXFS authors could clarify this?

The memcpy in question copies a c_node to the page. The len is either
the max length of a c_node and size of the buffer I'm copying to
(PAGE_CACHE_SIZE) or it is the difference between the beginning of the
c_node in the c_block and the end of the c_block, whichever is
smaller. The confusion is probably because of the fact that this
copies extra crap to the page for tails.