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SubjectRe: [PATCH] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo (resend)
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On Thu, 2008-08-21 at 14:28 +0100, Mel Gorman wrote:
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 4b8acd2..70dba16 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -810,6 +810,11 @@ config OABI_COMPAT
> UNPREDICTABLE (in fact it can be predicted that it won't work
> at all). If in doubt say Y.
>
> +config ARCH_FLATMEM_HAS_HOLES
> + bool
> + default y
> + depends on FLATMEM
> +
> config ARCH_DISCONTIGMEM_ENABLE
> bool
> default (ARCH_LH7A40X && !LH7A40X_CONTIGMEM)
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index b0d08e6..98aa882 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -516,9 +516,26 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
> continue;
>
> page = pfn_to_page(pfn);
> +#ifdef CONFIG_ARCH_FLATMEM_HAS_HOLES
> + /*
> + * Ordinarily, memory holes in flatmem still have a valid
> + * memmap for the PFN range. However, an architecture for
> + * embedded systems (e.g. ARM) can free up the memmap backing
> + * holes to save memory on the assumption the memmap is
> + * never used.

I'm not sure where this assumption is coming from. Have you taken a
look at the ARM code?

> The page_zone linkages are then broken even
> + * though pfn_valid() returns true. Skip the page if the
> + * linkages are broken. Even if this test passed, the impact
> + * is that the counters for the movable type are off but
> + * fragmentation monitoring is likely meaningless on small
> + * systems.
> + */
> + if (page_zone(page) != zone)
> + continue;
> +#endif
> mtype = get_pageblock_migratetype(page);
>
> - count[mtype]++;
> + if (mtype < MIGRATE_TYPES)
> + count[mtype]++;
> }

Is that really worth an #ifdef? It is in code that isn't in a hot path,
and the page_zone() should be a repetitive operation on a structure that
is already (or will soon be) in the cpu cache.

Ugh, but as I think about it, this is going to be a much more widespread
problem. Looking at the arm code, the memory for the mem_map[]e *does*
get freed back into the bootmem allocator:

free_memmap(int node, unsigned long start_pfn, unsigned long end_pfn)
{
...
if (pg < pgend)
free_bootmem_node(NODE_DATA(node), pg, pgend - pg);
}

Doesn't that mean that we can get random gunk in the middle of the
mem_map[] when someone else allocates and uses this memory?

We have quite a few things across the kernel that iterate over pfn
ranges and require pfn_valid() to be working.

Can we talk ARM into converting over to sparsemem? A plain sparsemem
port for MIPS that got coded up recently was ~25 lines of arch code. A
bit more if they decide to do vmemmap. There's even a nice bit for
Documentation/ about what was done:

http://article.gmane.org/gmane.linux.ports.mips.general/21248/match=mips+sparsemem

-- Dave



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