From mboxrd@z Thu Jan  1 00:00:00 1970
From: u.kleine-koenig@pengutronix.de (Uwe =?iso-8859-1?Q?Kleine-K=F6nig?=)
Date: Thu, 1 Dec 2011 21:55:37 +0100
Subject: does non-continuous RAM means I need to select the sparse memory
 model?
In-Reply-To: <20111201182852.GC9581@n2100.arm.linux.org.uk>
References: <20111129203010.GA26618@pengutronix.de>
 <20111201182852.GC9581@n2100.arm.linux.org.uk>
Message-ID: <20111201205537.GO26618@pengutronix.de>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Thu, Dec 01, 2011 at 06:28:52PM +0000, Russell King - ARM Linux wrote:
> On Tue, Nov 29, 2011 at 09:30:10PM +0100, Uwe Kleine-K?nig wrote:
> > I'm currently working on a new arch port and my current machine has RAM
> > at 0x10000000 and 0x80000000. So there is a big hole between the two
> > banks. When selecting the sparse memory model it works, but when
> > selecting flat the machine runs into a BUG in mark_bootmem() called by
> > free_unused_memmap() to free the space between the two banks.
> 
> Well, it looks like you have the answer in your latest email:
> 
> > Apart from that it seems I cannot use flatmem as is on my machine. It
> > has only 128kiB at 0x10000000 + 1MiB at 0x80000000 and needs 14MiB to hold the
> > table of "struct page"s.
> 
> Which shows that there's no possible way to use flatmem in this platform.
> 
> Now, to use sparsemem with this, sparsemem people will tell you that
> each sparsemem section must be fully populated.  So, as your minimum
> memory size is 128kiB, your sparsemem section size must be 128kiB.
> This makes SECTION_SIZE_BITS be 17.
> 
> You need to span to 0x80000000+, which means you need MAX_PHYSMEM_BITS
> to be 32.
> 
> These two together means you're in for having an array of 2^(32 - 17) =
> 32768 pointers to struct section for starters.  That takes 128kiB.
> Now add in the struct page structures, which for 128kiB of memory will
> be 1kiB.  Now add 9 struct section's as well...
> 
> What I'm pointing out is that in the scenario you appear to have, trying
> to use the small pool of memory which is soo dislocated from the rest of
> memory is a waste of effort.  Adding it to the pool means that you require
> more memory for the management data structures than you gain from adding
> it to the system.  The net benefit from trying to use it is actually
> negative.
> 
> So, forget that the 128kiB chunk of memory exists - at that size and
> distance from the rest of memory its really not worth bothering with.
many thanks for your description. I'll go with flatmem skipping the
128kiB bank now.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |