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    SubjectRe: Weirdness in __alloc_bootmem_node_high
    On Fri 20-04-12 21:14:18, Sam Ravnborg wrote:
    > On Fri, Apr 20, 2012 at 11:29:07AM -0700, Tejun Heo wrote:
    > > On Tue, Apr 17, 2012 at 05:55:02PM +0200, Michal Hocko wrote:
    > > > Hi,
    > > > I just come across the following condition in __alloc_bootmem_node_high
    > > > which I have hard times to understand. I guess it is a bug and we need
    > > > something like the following. But, to be honest, I have no idea why we
    > > > care about those 128MB above MAX_DMA32_PFN.
    > > > ---
    > > > mm/bootmem.c | 2 +-
    > > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > > >
    > > > diff --git a/mm/bootmem.c b/mm/bootmem.c
    > > > index 0131170..5adb072 100644
    > > > --- a/mm/bootmem.c
    > > > +++ b/mm/bootmem.c
    > > > @@ -737,7 +737,7 @@ void * __init __alloc_bootmem_node_high(pg_data_t *pgdat, unsigned long size,
    > > > /* update goal according ...MAX_DMA32_PFN */
    > > > end_pfn = pgdat->node_start_pfn + pgdat->node_spanned_pages;
    > > >
    > > > - if (end_pfn > MAX_DMA32_PFN + (128 >> (20 - PAGE_SHIFT)) &&
    > > > + if (end_pfn > MAX_DMA32_PFN + (128 << (20 - PAGE_SHIFT)) &&
    > > > (goal >> PAGE_SHIFT) < MAX_DMA32_PFN) {
    > > > void *ptr;
    > > > unsigned long new_goal;
    > >
    > > Regardless of x86 not using it, this is a bug fix and this code path
    > > seems to be used by mips at least.
    >
    > I took a quick look at this.
    > __alloc_bootmem_node_high() is used in mm/sparse.c - but only
    > if SPARSEMEM_VMEMMAP is enabled.

    This is what I can see in the current (Linus) git:
    ./arch/sparc/Kconfig: select SPARSEMEM_VMEMMAP_ENABLE
    ./arch/powerpc/Kconfig: select SPARSEMEM_VMEMMAP_ENABLE
    ./arch/ia64/Kconfig: select SPARSEMEM_VMEMMAP_ENABLE
    ./arch/s390/Kconfig: select SPARSEMEM_VMEMMAP_ENABLE
    ./arch/s390/Kconfig: select SPARSEMEM_VMEMMAP
    ./arch/x86/Kconfig: select SPARSEMEM_VMEMMAP_ENABLE if X86_64

    So there are more arches which enable SPARSEMEM_VMEMMAP so the function
    is used. Or am I missing something?

    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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