Re: [RFC 0/8] Variable Order Page Cache

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Apr 20 2007, David Chinner wrote:
> > - Higher order pages in the block layer etc.
> 
> It's more drivers that we have to worry about, I think.  We don't need to
> modify bios to explicitly support compound pages. From bio.h:
> 
> /*
>  * was unsigned short, but we might as well be ready for > 64kB I/O pages
>  */
> struct bio_vec {
>         struct page     *bv_page;
>         unsigned int    bv_len;
>         unsigned int    bv_offset;
> };
> 
> So compound pages should be transparent to anything that doesn't
> look at the contents of bio_vecs....

That just means you don't have to modify the bio_vec, there's still some
work to be done. But it should not be too hard, it's mainly updating the
merging checks. And grep for where PAGE_SIZE or PAGE_CACHE_SIZE is used
in fs/bio.c include/linux/bio.h block/ll_rw_blk.c

> > The ramfs driver can be used to test higher order page cache functionality
> > (and may help troubleshoot the VM support until we get some real filesystem
> > and real devices supporting higher order pages).
> 
> I don't think it will take much to get XFS to work with a high order
> page cache and we can probably insulate the block layer initially with some
> kind of bio_add_compound_page() wrapper and some similar
> wrapper on the io completion side.

Eh no way, at least not if you want it merged. Lets not repeat the XFS
kiobuf IO disaster :-). If this is to be done and merged, it needs to be
integrated nicely with the current framework, not attached to the side.

-- 
Jens Axboe