Ok, I analyzed the code and it seems that this alignment problem has
been solved by the changes made
to the free_unused_memmap() code in arch/arm/mm/init.c.

I backported those changes to free_unused_memmap_node() in
linux-2.6.35.9 and I don't see any more
crashes. This solves my problem.

Thanks for all the help.


On Fri, Aug 5, 2011 at 2:17 PM, Mel Gorman [off-list ref] wrote:

On Fri, Aug 05, 2011 at 11:27:21AM +0530, Kautuk Consul wrote:

quoted

Hi Mel,

Please find my comments inline to the email below.

2 general questions:
i)    If an email chain such as this leads to another kernel patch for
the same problem, do I need to
      create a new email chain for that ?

Yes. I believe it's easier on the maintainer if a new thread is started
with the new patch leader summarising relevant information from the
discussion. A happy maintainer makes the day easier.

quoted

ii)  Sorry about my formatting problems. However, text such as
backtraces and logs tend to wrap
      irrespective of whatever gmail settings/browser I try. Any
pointers here ?

I don't use gmail so I do not have any suggestions other than using a
different mailer. There are no suggestions in
Documentation/email-clients.txt on how to deal with gmail.

quoted

On Thu, Aug 4, 2011 at 3:39 PM, Mel Gorman [off-list ref] wrote:

quoted

On Thu, Aug 04, 2011 at 03:06:39PM +0530, Kautuk Consul wrote:

quoted

Hi Mel,

My ARM system has 2 memory banks which have the following 2 PFN ranges:
60000-62000 and 70000-7ce00.

My SECTION_SIZE_BITS is #defined to 23.

So bank 0 is 4 sections and bank 1 is 26 sections with the last section
incomplete.

quoted

I am altering the ranges via the following kind of pseudo-code in the
arch/arm/mach-*/mach-*.c file:
meminfo->bank[0].size -= (1 << 20)
meminfo->bank[1].size -= (1 << 20)

Why are you taking 1M off each bank? I could understand aligning the
banks to a section size at least.

The reason I am doing this is that one of our embedded boards actually
has this problem, due
to which we see this kernel crash. I am merely reproducing this
problem by performing this step.

Ah, that makes sense.

quoted

quoted

quoted

<SNIP>
The reason why we cannot expect the 0x61fff end_page->flags to contain
a valid zone number is:
memmap_init_zone() initializes the zone number of all pages for a zone
via the set_page_links() inline function.
For the end_page (whose PFN is 0x61fff), set_page_links() cannot be
possibly called, as the zones are simply not aware of of PFNs above
0x61f00 and below 0x70000.

Can you ensure that the ranges passed into free_area_init_node()
are MAX_ORDER aligned as this would initialise the struct pages. You
may have already seen that care is taken when freeing memmap that it
is aligned to MAX_ORDER in free_unused_memmap() in ARM.

Will this work ? My doubt arises from the fact that there is only one
zone on the entire
system which contains both memory banks.

That is a common situation. It's why present_pages and spanned_pages
in a zone can differ. As long as valid memmap is MAX_ORDER-aligned, it's
fine.

quoted

The crash arises at the PFN 0x61fff, which will not be covered by such
a check, as this function

No, it won't be covered by the range check. However, the memmap will be
initialised so even though the page is outside a valid bank of memory,
it'll still resolve to the correct zone. The struct page will be marked
Reserved so it'll never be used.

quoted

will try to act on the entire zone, which is the PFN range:
60000-7cd00, including the holes within as
all of this RAM falls into the same node and zone.
( Please correct me if I am wrong about this. )

I tried aligning the end parameter in the memory_present() function
which is called separately
for each memory bank.
I tried the following change in memory_present() as well as
mminit_validate_memodel_limits():
end &= ~(pageblock_nr_pages-1);
But, in this case, the board simply does not boot up. I think that
will then require some change in the
arch/arm code which I think would be an arch-specific solution to a
possibly generic problem.

I do not believe this is a generic problem. Machines have
holes in the zone all the time and this bug does not trigger.
mminit_validate_memodel_limits() is the wrong place to make a change.
Look more towards where free_area_init_node() gets called to initialise
memmap.

quoted

quoted

quoted

The (end >= zone->zone_start_pfn + zone->spanned_pages) in
move_freepages_block() does not stop this crash from happening as both
our memory banks are in the same zone and the empty space within them
is accomodated into this zone via the CONFIG_SPARSEMEM
config option.

When we enable CONFIG_HOLES_IN_ZONE we survive this BUG_ON as well as
any other BUG_ONs in the loop in move_freepages() as then the
pfn_valid_within()/pfn_valid() function takes care of this
functionality, especially in the case where the newly introduced
CONFIG_HAVE_ARCH_PFN_VALID is
enabled.

This is an expensive option in terms of performance. If Russell
wants to pick it up, I won't object but I would strongly suggest that
you solve this problem by ensuring that memmap is initialised on a
MAX_ORDER-aligned boundaries as it'll perform better.

I couldn't really locate a method in the kernel wherein we can
validate a pageblock(1024 pages for my
platform) with respect to the memory banks on that system.

I suspect what you're looking for is somewhere in arch/arm/mm/init.c .
I'm afraid I didn't dig through the ARM memory initialisation
code to see where it could be done but if it was me, I'd be looking at
how free_area_init_node() is called.

quoted

How about this :
We implement an arch_is_valid_pageblock() function, controlled by a
new config option
CONFIG_ARCH_HAVE IS_VALID_PAGEBLOCK.
This arch function will simply check whether this pageblock is valid
or not, in terms of arch-specific
memory banks or by using the memblock APIs depending on CONFIG_HAVE_MEMBLOCK.
We can modify the memmap_init_zone() function so that an outer loop
works in measures of
pageblocks thus enabling us to avoid invalid pageblocks.

That seems like massive overkill for what should be an alignment problem
when initialisating memmap. It's adding complexity that is similar to
HOLES_IN_ZONE with very little gain.

When it gets down to it, I'd even prefer deleting the BUG_ON as
the PageBuddy check over such a solution. However, the BUG_ON is
there because alignment to MAX_ORDER is expected so it is a valid
sanity check. There would need to be good evidence that initialising
memmap to MAX_ORDER alignment was somehow impossible.

--
Mel Gorman
SUSE Labs

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