Re: [00/17] Large Blocksize Support V3

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 27 Apr 2007 12:27:31 +1000 David Chinner <dgc@sgi.com> wrote:

> On Thu, Apr 26, 2007 at 07:04:38PM -0700, Andrew Morton wrote:
> > On Tue, 24 Apr 2007 15:21:05 -0700 clameter@sgi.com wrote:
> > 
> > > This patchset modifies the Linux kernel so that larger block sizes than
> > > page size can be supported. Larger block sizes are handled by using
> > > compound pages of an arbitrary order for the page cache instead of
> > > single pages with order 0.
> > 
> > Something I was looking for but couldn't find: suppose an application takes
> > a pagefault against the third 4k page of an order-2 pagecache "page".  We
> > need to instantiate a pte against find_get_page(offset/4)+3.  But these
> > patches don't touch mm/memory.c at all and filemap_nopage() appears to
> > return the zeroeth 4k page all the time in that case.
> > 
> > So.. what am I missing, and how does that part work?
> 
> "mmap not supported yet" ;)

erk.  I suspect this will have its sticky paws all over core mm.


> > Also, afaict your important requirements would be met by retaining
> > PAGE_CACHE_SIZE=4k and simply ensuring that pagecache is populated by
> > physically contiguous pages
> 
> Sure, that addresses the larger I/O side of things, but it doesn't address
> the large filesystem blocksize issues that can only be solved with some kind
> of page aggregation abstraction.

a) That wasn't a part of Christoph's original rationale list, so forgive
   me for thinking it is not so important and got snuck in post-facto when
   things got tough.

b) I don't immediately see why a filesystam cannot implement larger
   blocksizes via this scheme - instantiate and lock four pages and go for
   it.

> Compound pages and high order page cache
> indexing solves this extremely neatly, regardless of whether the compound
> page is contiguous or not.....

We cannot say anything about neatness until we've seen mmap.