linux-kernel - Re: [Lsf-pc] [LSF/MM TOPIC] really large storage sectors

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Date:	Thu, 30 Jan 2014 17:01:03 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Mel Gorman <mgorman@...e.de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Chris Mason <clm@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"lsf-pc@...ts.linux-foundation.org" 
	<lsf-pc@...ts.linux-foundation.org>,
	"rwheeler@...hat.com" <rwheeler@...hat.com>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] really large storage sectors - going
 beyond 4096 bytes

On Wed, Jan 29, 2014 at 09:52:46PM -0700, Matthew Wilcox wrote:
> On Fri, Jan 24, 2014 at 10:57:48AM +0000, Mel Gorman wrote:
> > So far on the table is
> > 
> > 1. major filesystem overhawl
> > 2. major vm overhawl
> > 3. use compound pages as they are today and hope it does not go
> >    completely to hell, reboot when it does
> 
> Is the below paragraph an exposition of option 2, or is it an option 4,
> change the VM unit of allocation?  Other than the names you're using,
> this is basically what I said to Kirill in an earlier thread; either
> scrap the difference between PAGE_SIZE and PAGE_CACHE_SIZE, or start
> making use of it.

Christoph Lamater's compound page patch set scrapped PAGE_CACHE_SIZE
and made it a variable that was set on the struct address_space when
it was instantiated by the filesystem. In effect, it allowed
filesystems to specify the unit of page cache allocation on a
per-inode basis.

> The fact that EVERYBODY in this thread has been using PAGE_SIZE when they
> should have been using PAGE_CACHE_SIZE makes me wonder if part of the
> problem is that the split in naming went the wrong way.  ie use PTE_SIZE
> for 'the amount of memory pointed to by a pte_t' and use PAGE_SIZE for
> 'the amount of memory described by a struct page'.

PAGE_CACHE_SIZE was never distributed sufficiently to be used, and
if you #define it to something other than PAGE_SIZE stuff will
simply break.

> (we need to remove the current users of PTE_SIZE; sparc32 and powerpc32,
> but that's just a detail)
> 
> And we need to fix all the places that are currently getting the
> distinction wrong.  SMOP ... ;-)  What would help is correct typing of
> variables, possibly with sparse support to help us out.  Big Job.

Yes, that's what the Christoph's patchset did.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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