linux-kernel - Re: [RFC PATCH] Support map

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Message-ID: <20140317114321.GA30191@node.dhcp.inet.fi>
Date:	Mon, 17 Mar 2014 13:43:21 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Toshi Kani <toshi.kani@...com>, kirill.shutemov@...ux.intel.com,
	david@...morbit.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH] Support map_pages() for DAX

On Sat, Mar 15, 2014 at 10:46:13PM -0400, Matthew Wilcox wrote:
> On Sat, Mar 15, 2014 at 01:32:33AM +0200, Kirill A. Shutemov wrote:
> > Side note: I'm sceptical about whole idea to use i_mmap_mutux to protect
> > against truncate. It will not scale good enough comparing lock_page()
> > with its granularity.
> 
> I'm actually working on this now.  The basic idea is to put an entry in
> the radix tree for each page.  For zero pages, that's a pagecache page.
> For pages that map to the media, it's an exceptional entry.  Radix tree
> exceptional entries take two bits, leaving us with 30 or 62 bits depending
> on sizeof(void *).  We can then take two more bits for Dirty and Lock,
> leaving 28 or 60 bits that we can use to cache the PFN on the page,
> meaning that we won't have to call the filesystem's get_block as often.

Sound reasonable to me. Implementation of ->map_pages should be trivial
with this.

Few questions:
 - why would you need Dirty for DAX?
 - are you sure that 28 bits is enough for PFN everywhere?
   ARM with LPAE can have up to 40 physical address lines. Is there any
   32-bit machine with more address lines?

-- 
 Kirill A. Shutemov
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