[Btrfs-devel] Re: [ANNOUNCE] Btrfs v0.12 released

[Chris Mason]

On Tuesday 12 February 2008, David Miller wrote:
> From: Chris Mason <chris.mason at oracle.com>
> Date: Mon, 11 Feb 2008 08:42:20 -0500
>
> > The kernel is actually worse, because the set/get macros are more
> > complex. Some live in ctree.h like in the progs, but the nasty ones live
> > in struct-funcs.c
>
> This is really problematic, because you've got these things called
> "btrfs_item_ptr()" which really isn't a pointer, it's a relative
> 'unsigned long' offset cast to a pointer.  The source of this
> seems to be btrfs_leaf_data().
>
> And then those things get passed down into the SETGET functions!

Explaining it won't make it pretty, but at least I can tell you what the code 
does.

This is all part of the btrfs code that supports tree block sizes larger than 
a page.  The extent_buffer code (extent_io.c) provides a read/write api into 
an extent_buffer based on offsets from the start of the multi-page buffer.  
That's where the relative unsigned long comes from.

The part where I cast it to pointers is me trying to maintain type checking 
throughout the code.  The pointers themselves are useless, they need to be 
matched with an extent_buffer to actually get to the bytes.

There are a few parts where the SETGET funcs are open coded, mostly in very 
performance critical functions.  Just look for lexxx_to_cpu

>
> Then deeper down we have terribly inconsistent things like
> btrfs_item_nr_offset() and
> btrfs_item_offset_nr().

Btree blocks have the offset of the item header from the start of the block 
and the offset of the item data.  And, I'm very bad at naming.

>
> Sigh... I'll see what I can do.

Thanks

-chris