On Mon, Jun 25, 2007 at 05:41:58PM +1000, Nick Piggin wrote:
> Neil Brown wrote:
> >On Sunday June 24, [email protected] wrote:
> >
> >>
> >>+#define PG_blocks 20 /* Page has block mappings */
> >>+
> >
> >
> >I've only had a very quick look, but this line looks *very* wrong.
> >You should be using PG_private.
> >
> >There should never be any confusion about whether ->private has
> >buffers or blocks attached as the only routines that ever look in
> >->private are address_space operations (or should be. I think 'NULL'
> >is sometimes special cased, as in try_to_release_page. It would be
> >good to do some preliminary work and tidy all that up).
>
> There is a lot of confusion, actually :)
> But as you see in the patch, I added a couple more aops APIs, and
> am working toward decoupling it as much as possible. It's pretty
> close after the fsblock patch... however:
>
>
> >Why do you think you need PG_blocks?
>
> Block device pagecache (buffer cache) has to be able to accept
> attachment of either buffers or blocks for filesystem metadata,
> and call into either buffer.c or fsblock.c based on that.
>
> If the page flag is really important, we can do some awful hack
> like assuming the first long of the private data is flags, and
> those flags will tell us whether the structure is a buffer_head
> or fsblock ;) But for now it is just easier to use a page flag.
The block device pagecache isn't special, and certainly isn't that much
code. I would suggest keeping it buffer head specific and making a
second variant that does only fsblocks. This is mostly to keep the
semantics of PagePrivate sane, lets not fuzz the line.
-chris
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