Messages in this thread | | | Subject | Re: [PATCH] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo (resend) | From | Dave Hansen <> | Date | Fri, 22 Aug 2008 09:19:14 -0700 |
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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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