Messages in this thread | | | Date | Wed, 6 May 2009 17:50:43 +0200 | From | Johannes Weiner <> | Subject | Re: [PATCH] Double check memmap is actually valid with a memmap has unexpected holes |
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On Wed, May 06, 2009 at 03:31:00PM +0100, Mel Gorman wrote: > On Tue, May 05, 2009 at 01:06:53PM +0200, Johannes Weiner wrote: > > Hi Mel, > > > > On Tue, May 05, 2009 at 09:29:44AM +0100, Mel Gorman wrote: > > > pfn_valid() is meant to be able to tell if a given PFN has valid memmap > > > associated with it or not. In FLATMEM, it is expected that holes always > > > have valid memmap as long as there is valid PFNs either side of the hole. > > > In SPARSEMEM, it is assumed that a valid section has a memmap for the > > > entire section. > > > > > > However, ARM and maybe other embedded architectures in the future free > > > memmap backing holes to save memory on the assumption the memmap is never > > > used. The page_zone() linkages are then broken even though pfn_valid() > > > returns true. A walker of the full memmap in this case must do additional > > > check to ensure the memmap they are looking at is sane by making sure the > > > zone and PFN linkages are still valid. This is expensive, but walkers of > > > the full memmap are extremely rare. > > > > > > This was caught before for FLATMEM and hacked around but it hits again > > > for SPARSEMEM because the page_zone() linkages can look ok where the PFN > > > linkages are totally screwed. This looks like a hatchet job but the reality > > > is that any clean solution would end up consuming all the memory saved > > > by punching these unexpected holes in the memmap. For example, we tried > > > marking the memmap within the section invalid but the section size exceeds > > > the size of the hole in most cases so pfn_valid() starts returning false > > > where valid memmap exists. Shrinking the size of the section would increase > > > memory consumption offsetting the gains. > > > > > > This patch identifies when an architecture is punching unexpected holes > > > in the memmap that the memory model cannot automatically detect. When set, > > > walkers of the full memmap must call memmap_valid_within() for each PFN and > > > passing in what it expects the page and zone to be for that PFN. If it finds > > > the linkages to be broken, it assumes the memmap is invalid for that PFN. > > > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie> > > > > I think we also need to fix up show_mem(). > > As it turns out, ARM has its own show_mem(). I don't see how, but ARM > must not be using lib/show_mem.c even though it compiles it.
It's some linker magic for lib/. It compiles both but treats the library version as weak symbol (or something).
But with the zone-walking show_mem, I think it should be able to use the generic version.
> > Attached is a > > compile-tested patch, please have a look. I am not sure about memory > > hotplug issues but on a quick glance the vmstat stuff seems to be > > optimistic as well. > > > > --- > > From: Johannes Weiner <hannes@cmpxchg.org> > > Subject: lib: adjust show_mem() to support memmap holes > > > > Some architectures free the backing of mem_map holes. pfn_valid() is > > not able to report this properly, so a stronger check is needed if the > > caller is about to use the page descriptor derived from a pfn. > > > > Change the node walking to zone walking and use memmap_valid_within() > > to check for holes. This is reliable as it additionally checks for > > page_zone() and page_to_pfn() coherency. > > > > Not-yet-signed-off-by: Johannes Weiner <hannes@cmpxchg.org> > > --- > > lib/show_mem.c | 21 +++++++++------------ > > 1 files changed, 9 insertions(+), 12 deletions(-) > > > > diff --git a/lib/show_mem.c b/lib/show_mem.c > > index 238e72a..ed3c3ec 100644 > > --- a/lib/show_mem.c > > +++ b/lib/show_mem.c > > @@ -11,29 +11,27 @@ > > > > void show_mem(void) > > { > > - pg_data_t *pgdat; > > unsigned long total = 0, reserved = 0, shared = 0, > > nonshared = 0, highmem = 0; > > + struct zone *zone; > > > > printk(KERN_INFO "Mem-Info:\n"); > > show_free_areas(); > > > > - for_each_online_pgdat(pgdat) { > > - unsigned long i, flags; > > + for_each_populated_zone(zone) { > > + unsigned long start = zone->zone_start_pfn; > > + unsigned long end = start + zone->spanned_pages; > > The patch appears to be doing two things > > o Scanning zones instead of pgdats > o Adding the use of memmap_valid_within() > > Scanning zones instead of pgdats seems like a good idea on its own and should > be split out for separate consideration.
Good idea. Will do.
> > + unsigned long pfn; > > > > - pgdat_resize_lock(pgdat, &flags); > > How sure are you about removing the acquisition of this lock? If anything, > it appears that pagetypeinfo_showblockcount_print() should be taking this lock.
I'm completely unsure about it.
<adds memory hotplug guys to CC>
> > - for (i = 0; i < pgdat->node_spanned_pages; i++) { > > - struct page *page; > > - unsigned long pfn = pgdat->node_start_pfn + i; > > + for (pfn = start; pfn < end; pfn++) { > > + struct page *page = pfn_to_page(pfn); > > > > You need to check pfn_valid() before using pfn_to_page(). > > > - if (unlikely(!(i % MAX_ORDER_NR_PAGES))) > > + if (unlikely(!(pfn % MAX_ORDER_NR_PAGES))) > > touch_nmi_watchdog(); > > > > - if (!pfn_valid(pfn)) > > + if (!memmap_valid_within(pfn, page, zone)) > > continue; > > > > You need both the pfn_valid() check and the memmap_valid_within() as > memmap_valid_within() unconditionally returns 1 for most architectures. If > you applied this patch as-is, memory holes in a zone will cause big problems - > random results at best and invalid memory references at worst.
Ah, zone holes. More reading up needed. Thanks, Mel.
Hannes
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