[Crash-utility] Incorrect HighMem statistics

[Dave Anderson]

----- Original Message -----
> Dne Čt 12. ledna 2012 22:38:38 Petr Tesarik napsal(a):
> > Hi all,
> > 
> > it seems crash shows incorrect HighMem stats with recent kernels.
> > E.g.:
> > 
> > crash> kmem -i
> > [...]
> > TOTAL HIGH  1821682       6.9 GB   93% of TOTAL MEM
> >  FREE HIGH        0            0    0% of TOTAL HIGH
> >  TOTAL LOW   132983     519.5 MB    6% of TOTAL MEM
> >   FREE LOW  1333474       5.1 GB  1002% of TOTAL LOW
> > 
> > BTW note that total low is smaller than free low, which is obviously
> > incorrect. I believe that this is somehow related to the Movable zone,
> > because the code that counts free low pages checks for pages which belong
> > to ZONE_HIGHMEM, which is initialized as:
> > 
> >                 vt->ZONE_HIGHMEM = vt->nr_zones - 1;
> > 
> > My system has 4 zones:
> > 
> > ZONE  NAME         SIZE   MEM_MAP  START_PADDR  START_MAPNR
> >   0   DMA          4080  f4a02200        10000            0
> >   1   Normal     221694  f4a22000      1000000         4080
> >   2   HighMem    790002  f50e5fc0     371fe000       225774
> >   3   Movable         0         0            0            0
> > 
> > And indeed, "help -v" shows:
> > [...]
> >     dump_free_pages: dump_free_pages_zones_v2()
> > [...]
> >        ZONE_HIGHMEM: 3
> > 
> > I don't know yet how to fix this, but maybe somebody can push me in
> > the
> > right direction.
> 
> Maybe we should check the names in zone_names[]. AFAICS this array has existed
> ever since the dawn of time, and the HIGHMEM_ZONE has always been called
> "HighMem".

I agree -- I'm sure my original intent was based upon paranoia that it
might change.
 
> That already covers a lot of cases, but not the Movable zone, if it exists.
> nr_free_highpages() contains:
> 
>                 if (zone_movable_is_highmem())
>                         pages += zone_page_state(
>                                         &pgdat->node_zones[ZONE_MOVABLE],
>                                         NR_FREE_PAGES);
> 
> zone_movable_is_highmem() is defined as:
> 
> static inline int zone_movable_is_highmem(void)
> {
> #if defined(CONFIG_HIGHMEM) &&
> defined(CONFIG_ARCH_POPULATES_NODE_MAP)
>         return movable_zone == ZONE_HIGHMEM;
> #else
>         return 0;
> #endif
> }
> 
> Now, I think we don't have to take the preprocessor directives into account,
> because movable_zone is automatically initialized to zero, and ZONE_HIGHMEM
> can never be numerically equal to zero, so the test would be always false in
> that case anyway.

Maybe you could first determine whether the kernel symbol "movable_zone" exists, 
and if so, what it contains:

  ...
  int movable_zone;
  EXPORT_SYMBOL(movable_zone);
  #endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */

> 
> We should only set vt->HIGHMEM_ZONE to something special (such as -1) if
> CONFIG_HIGHMEM is not defined, and there is no highmem zone, because otherwise
> we might hit a false positive for zone_movable_is_highmem.
> 
> Comments? Should I make the idea into a patch?

Absolutely -- I'm waiting...    ;-)

Thanks,
  Dave