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

[David Chinner <dgc@sgi.com>]

On Thu, Apr 19, 2007 at 09:47:18PM -0700, William Lee Irwin III wrote:
> On Thu, Apr 19, 2007 at 09:35:04AM -0700, Christoph Lameter wrote:
> > This patchset modifies the core VM so that higher order page cache pages
> > become possible. The higher order page cache pages are compound pages
> > and can be handled in the same way as regular pages.
> > The order of the pages is determined by the order set up in the mapping
> > (struct address_space). By default the order is set to zero.
> > This means that higher order pages are optional. There is no attempt here
> > to generally change the page order of the page cache. 4K pages are effective
> > for small files.
> 
> Oh dear. Per-file pagesizes are foul. Better to fix up the pagecache's
> radix tree than to restrict it like this.

How is this restrictive? This opens up much goodness to filesystems.
I *want* to be able to use different radix tree index orders for
different inodes in a single filesystem. Not just for data, either;
XFS has metadata constructs larger than a page that mean we have to
carry our own buffer cache around.  Being able to use the page cache
directly for these metadata constructs would enable us to remove a
good chunk of code from XFS....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group