Re: [PATCH 3/5] x86: Only direct map addresses that are marked asE820_RAM

[Jacob Shin]

On Fri, Aug 24, 2012 at 09:54:04PM -0700, Yinghai Lu wrote:
> On Fri, Aug 24, 2012 at 9:24 PM, Jacob Shin <jacob.shin@xxxxxxx> wrote:
> > On Fri, Aug 24, 2012 at 06:07:01PM -0700, Yinghai Lu wrote:
> >> On Fri, Aug 24, 2012 at 4:55 PM, Jacob Shin <jacob.shin@xxxxxxx> wrote:
> >> > Currently direct mappings are created for [ 0 to max_low_pfn<<PAGE_SHIFT )
> >> > and [ 4GB to max_pfn<<PAGE_SHIFT ), which may include regions that are not
> >> > backed by actual DRAM. This is fine for holes under 4GB which are covered
> >> > by fixed and variable range MTRRs to be UC. However, we run into trouble
> >> > on higher memory addresses which cannot be covered by MTRRs.
> >> >
> >> > Our system with 1TB of RAM has an e820 that looks like this:
> >> >
> >> >  BIOS-e820: [mem 0x0000000000000000-0x00000000000983ff] usable
> >> >  BIOS-e820: [mem 0x0000000000098400-0x000000000009ffff] reserved
> >> >  BIOS-e820: [mem 0x00000000000d0000-0x00000000000fffff] reserved
> >> >  BIOS-e820: [mem 0x0000000000100000-0x00000000c7ebffff] usable
> >> >  BIOS-e820: [mem 0x00000000c7ec0000-0x00000000c7ed7fff] ACPI data
> >> >  BIOS-e820: [mem 0x00000000c7ed8000-0x00000000c7ed9fff] ACPI NVS
> >> >  BIOS-e820: [mem 0x00000000c7eda000-0x00000000c7ffffff] reserved
> >> >  BIOS-e820: [mem 0x00000000fec00000-0x00000000fec0ffff] reserved
> >> >  BIOS-e820: [mem 0x00000000fee00000-0x00000000fee00fff] reserved
> >> >  BIOS-e820: [mem 0x00000000fff00000-0x00000000ffffffff] reserved
> >> >  BIOS-e820: [mem 0x0000000100000000-0x000000e037ffffff] usable
> >> >  BIOS-e820: [mem 0x000000e038000000-0x000000fcffffffff] reserved
> >> >  BIOS-e820: [mem 0x0000010000000000-0x0000011ffeffffff] usable
> >> >
> >> > and so direct mappings are created for huge memory hole between
> >> > 0x000000e038000000 to 0x0000010000000000. Even though the kernel never
> >> > generates memory accesses in that region, since the page tables mark
> >> > them incorrectly as being WB, our (AMD) processor ends up causing a MCE
> >> > while doing some memory bookkeeping/optimizations around that area.
> >> >
> >> > This patch iterates through e820 and only direct maps ranges that are
> >> > marked as E820_RAM, and keeps track of those pfn ranges. Depending on
> >> > the alignment of E820 ranges, this may possibly result in using smaller
> >> > size (i.e. 4K instead of 2M or 1G) page tables.
> >> >
> >> > Signed-off-by: Jacob Shin <jacob.shin@xxxxxxx>
> >> > ---
> >> >  arch/x86/include/asm/page_types.h |    9 +++
> >> >  arch/x86/kernel/setup.c           |  125 +++++++++++++++++++++++++++++--------
> >> >  arch/x86/mm/init.c                |    2 +
> >> >  arch/x86/mm/init_64.c             |    6 +-
> >> >  4 files changed, 112 insertions(+), 30 deletions(-)
> >> >
> >> > diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
> >> > index e21fdd1..409047a 100644
> >> > --- a/arch/x86/include/asm/page_types.h
> >> > +++ b/arch/x86/include/asm/page_types.h
> >> > @@ -3,6 +3,7 @@
> >> >
> >> >  #include <linux/const.h>
> >> >  #include <linux/types.h>
> >> > +#include <asm/e820.h>
> >> >
> >> >  /* PAGE_SHIFT determines the page size */
> >> >  #define PAGE_SHIFT     12
> >> > @@ -40,12 +41,20 @@
> >> >  #endif /* CONFIG_X86_64 */
> >> >
> >> >  #ifndef __ASSEMBLY__
> >> > +#include <linux/range.h>
> >> >
> >> >  extern int devmem_is_allowed(unsigned long pagenr);
> >> >
> >> >  extern unsigned long max_low_pfn_mapped;
> >> >  extern unsigned long max_pfn_mapped;
> >> >
> >> > +extern struct range pfn_mapped[E820_X_MAX];
> >> > +extern int nr_pfn_mapped;
> >> > +
> >> > +extern void add_pfn_range_mapped(unsigned long start_pfn, unsigned long end_pfn);
> >> > +extern bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn);
> >> > +extern bool pfn_is_mapped(unsigned long pfn);
> >> > +
> >> >  static inline phys_addr_t get_max_mapped(void)
> >> >  {
> >> >         return (phys_addr_t)max_pfn_mapped << PAGE_SHIFT;
> >> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> >> > index 751e020..4217fb4 100644
> >> > --- a/arch/x86/kernel/setup.c
> >> > +++ b/arch/x86/kernel/setup.c
> >> > @@ -115,13 +115,46 @@
> >> >  #include <asm/prom.h>
> >> >
> >> >  /*
> >> > - * end_pfn only includes RAM, while max_pfn_mapped includes all e820 entries.
> >> > - * The direct mapping extends to max_pfn_mapped, so that we can directly access
> >> > - * apertures, ACPI and other tables without having to play with fixmaps.
> >> > + * max_low_pfn_mapped: highest direct mapped pfn under 4GB
> >> > + * max_pfn_mapped:     highest direct mapped pfn over 4GB
> >> > + *
> >> > + * The direct mapping only covers E820_RAM regions, so the ranges and gaps are
> >> > + * represented by pfn_mapped
> >> >   */
> >> >  unsigned long max_low_pfn_mapped;
> >> >  unsigned long max_pfn_mapped;
> >> >
> >> > +struct range pfn_mapped[E820_X_MAX];
> >> > +int nr_pfn_mapped;
> >> > +
> >> > +void add_pfn_range_mapped(unsigned long start_pfn, unsigned long end_pfn)
> >> > +{
> >> > +       nr_pfn_mapped = add_range_with_merge(pfn_mapped, E820_X_MAX,
> >> > +                                            nr_pfn_mapped, start_pfn, end_pfn);
> >> > +
> >> > +       max_pfn_mapped = max(max_pfn_mapped, end_pfn);
> >> > +
> >> > +       if (end_pfn <= (1UL << (32 - PAGE_SHIFT)))
> >> > +               max_low_pfn_mapped = max(max_low_pfn_mapped, end_pfn);
> >> > +}
> >> > +
> >> > +bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn)
> >> > +{
> >> > +       int i;
> >> > +
> >> > +       for (i = 0; i < nr_pfn_mapped; i++)
> >> > +               if ((start_pfn >= pfn_mapped[i].start) &&
> >> > +                   (end_pfn <= pfn_mapped[i].end))
> >> > +                       return true;
> >> > +
> >> > +       return false;
> >> > +}
> >> > +
> >> > +bool pfn_is_mapped(unsigned long pfn)
> >> > +{
> >> > +       return pfn_range_is_mapped(pfn, pfn + 1);
> >> > +}
> >> > +
> >>
> >> looks like you could avoid add pfn_mapped[] array.
> >>
> >> pfn_range_is_mapped() should be
> >> check max_low_pfn_mapped, max_pfn_mapped with
> >> e820_all_mapped(start, end, E820_RAM).
> >
> > Hmm .. I guess that could work .. but what about EFI code that keys off of
> > EFI memory map? Does the EFI code update e820 and mark as E820_RAM whatever
> > ranges that it calls init_memory_mapping on (via efi_ioremap?)
> 
> they are converted to e820 memmap before init_memory_mapping is called.

Yinghai, another question, what about hotplug? Are we guaranteed that
we will always be adding memory above max_pfn_mapped? And hotplug will
also update e820 to mark the range as E820_RAM as well?

Thanks!

-Jacob

> 
> Thanks
> 
> Yinghai
> 

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