linux-kernel - Re: [00/17] Large Blocksize Support V3

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Message-Id: <20070426195357.597ffd7e.akpm@linux-foundation.org>
Date:	Thu, 26 Apr 2007 19:53:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Chinner <dgc@....com>
Cc:	clameter@....com, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

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