From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with ESMTP id F07036B008A for ; Thu, 1 Dec 2011 11:10:28 -0500 (EST) Received: by bke17 with SMTP id 17so3219743bke.14 for ; Thu, 01 Dec 2011 08:10:26 -0800 (PST) Message-ID: <4ED7A6EF.1000705@the2masters.de> Date: Thu, 01 Dec 2011 17:10:23 +0100 From: Stefan Hellermann MIME-Version: 1.0 Subject: Re: flatmem broken for nommu? [Was: Re: does non-continuous RAM means I need to select the sparse memory model?] References: <20111129203010.GA26618@pengutronix.de> <20111201105718.GJ26618@pengutronix.de> <20111201153933.GL26618@pengutronix.de> In-Reply-To: <20111201153933.GL26618@pengutronix.de> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: =?ISO-8859-1?Q?Uwe_Kleine-K=F6nig?= Cc: linux-mm@kvack.org, Yinghai Lu , akpm@linux-foundation.org, linux-arm-kernel@lists.infradead.org Am 01.12.2011 16:39, schrieb Uwe Kleine-Konig: > Hello, > > On Thu, Dec 01, 2011 at 11:57:18AM +0100, Uwe Kleine-Konig wrote: >> On Tue, Nov 29, 2011 at 10:39:10PM -0200, Fabio Estevam wrote: >>> 2011/11/29 Uwe Kleine-Konig : >>>> Hello, >>>> >>>> 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. >>> >>> My understanding is that you have to select ARCH_HAS_HOLES_MEMORYMODEL. >> I think that is not necessary. >> >>>> Is that expected (meaning I cannot use the flat model)? I currently >>>> don't have another machine handy that has >1 memory back to test that. >>> >>> In case you have access to a MX35PDK you can try on this board as it does have >>> the memory hole. >> No I havn't, but I just used a 128MB machine and changed that in the >> .fixup callback to 64MB + 32MB with a 32MB hole in between and it works >> fine without ARCH_HAS_HOLES_MEMORYMODEL. >> >> I debugged the problem a bit further and one symptom is that >> >> struct page *mem_map >> >> is NULL for me. That looks wrong. I guess this is just broken for nommu. >> I will dig into that later today. > The problem is that the memory for mem_map is allocated using: > > map = alloc_bootmem_node_nopanic(pgdat, size); > > without any error checking. The _nopanic was introduced by commit > > 8f389a99 (mm: use alloc_bootmem_node_nopanic() on really needed path) > > I don't understand the commit's log and don't really see why it should > be allowed to not panic if the allocation failes here but use a NULL > pointer instead. > I put the people involved in 8f389a99 on Cc, maybe someone can comment? > > Apart from that it seems I cannot use flatmem as is on my machine. It > has only 128kiB@0x10000000 + 1MiB@0x80000000 and needs 14MiB to hold the > table of "struct page"s. :-( > > Best regards > Uwe > The commit was made after an bug report from me. I have an old x86 tablet pc with only 8Mb Ram. This machine fails early on bootup without this commit. I found an archived message of the bug report here: http://comments.gmane.org/gmane.linux.kernel/1135909 Regards, Stefan -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org