linux-kernel - Re: [PATCH 04/10] AXFS: axfs

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Message-ID: <6934efce0808212023x758babf0w500da6801bd66f45@mail.gmail.com>
Date:	Thu, 21 Aug 2008 20:23:55 -0700
From:	"Jared Hulbert" <jaredeh@...il.com>
To:	"Phillip Lougher" <phillip@...gher.demon.co.uk>
Cc:	"Arnd Bergmann" <arnd@...db.de>, Linux-kernel@...r.kernel.org,
	linux-embedded@...r.kernel.org,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	"Jörn Engel" <joern@...fs.org>,
	tim.bird@...sony.com, cotte@...ibm.com, nickpiggin@...oo.com.au
Subject: Re: [PATCH 04/10] AXFS: axfs_inode.c

> 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.
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