linux-kernel - Re: [PATCH] Fix spurious BUG_ON() in mark

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Date:	Thu, 10 Jul 2008 00:43:42 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix spurious BUG_ON() in mark_bootmem()

Hi Lee,

Lee Schermerhorn <Lee.Schermerhorn@...com> writes:

> Against:  2.6.26-rc8-mm1
>
> Fixes problem introduced by patches:
>
> bootmem-factor-out-the-marking-of-a-pfn-range.patch
> bootmem-replace-node_boot_start-in-struct-bootmem_data.patch
>
> HP ia64 NUMA platform fails to boot 26-rc8-mm1, hitting BUG_ON()
> in mm/bootmem.c:mark_bootmem().
>
> After linking all bootmem chunks, the 'bdata_list' on HP ia64 numa
> platforms looks something like this:
>
>         node 4:  0x0-0x8000
>         node 0:  0x1c008000-0x1c07ec00
>         node 1:  0x1c800000-0x1c87f000
>         node 2:  0x1d000000-0x1d07f000
>         node 3:  0x1d800000-0x1d87f000
>
> [Node 4 is a pseudo-node generated by the platform firmware to
> contain a configurable amount of zero-based, hardware interleaved
> memory. 0x8000 pages or 512M is the minimum that can be configured.]
>
> First call to mark_bootmem() [from free_bootmem()] called with:
>
> 	start-end:  0x1c008063-0x1c008262, reserve: 0, flags: 0
>
> I.e, NOT in the first chunk of the list.
>
> However, the "if (pos < bdata->node_min_pfn)" in the loop fails
> to test the start address of the argument range [in 'pos'] against
> the end of the chunk.  So, it treats the range as being in the node
> 4 chunk.  Second time thru' the loop, pos == 0x8000 is <
> bdata->node_min_pfn and pos != start, so we trip the BUG_ON().
>
> This patch enhances the if condition to skip chunks that do not
> overlap the argument range, allowing 26-rc8-mm1 to boot on this
> platform.
>
> Signed-off-by: Lee Schermerhorn <lee.schermerhorn@...com>
>
>  mm/bootmem.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> Index: linux-2.6.26-rc8-mm1/mm/bootmem.c
> ===================================================================
> --- linux-2.6.26-rc8-mm1.orig/mm/bootmem.c	2008-07-09 16:11:23.000000000 -0400
> +++ linux-2.6.26-rc8-mm1/mm/bootmem.c	2008-07-09 16:13:46.000000000 -0400
> @@ -299,7 +299,8 @@ static int __init mark_bootmem(unsigned 
>  		int err;
>  		unsigned long max;
>  
> -		if (pos < bdata->node_min_pfn) {
> +		if (pos < bdata->node_min_pfn ||
> +		    pos >= bdata->node_low_pfn) {
>  			BUG_ON(pos != start);
>  			continue;
>  		}

Ah, yeah, this was obviously wrong.  Thanks for the fix!

	Acked-by: Johannes Weiner <hannes@...urebad.de>

Nice to know that it boots otherwise on such a setup :)

	Hannes
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