3.2.2 panics on a 16GB i686 blade:
BUG: unable to handle kernel paging request at 01c00008
IP: [<c0522399>] isolate_migratepages+0x119/0x390
*pdpt = 000000002f7ce001 *pde = 0000000000000000
The crash happens on this line in mm/compaction.c::isolate_migratepages:
328 page = pfn_to_page(low_pfn);
This macro finds the struct page pointer for a given pfn. These struct
page pointers are stored in sections of 131072 pages if
CONFIG_SPARSEMEM=y. If an entire section has no memory pages, the page
structs are not allocated for this section. On this particular machine,
there is no RAM mapped from 2GB - 4GB:
# dmesg|grep usable
BIOS-e820: 0000000000000000 - 000000000009f400 (usable)
BIOS-e820: 0000000000100000 - 000000007fe4e000 (usable)
BIOS-e820: 000000007fe56000 - 000000007fe57000 (usable)
BIOS-e820: 0000000100000000 - 000000047ffff000 (usable)
So there are no page structs for the sections between 2GB and 4GB.
I believe this check was intended to catch page numbers that point to holes:
323 if (!pfn_valid_within(low_pfn))
324 continue;
But pfn_valid_within is defined to (1) on all archs except ARM and ia64
as far as I can tell. So this check always passes (it's in fact
optimized out), and pfn_to_page ends up dereferencing an invalid address
due to a null pointer in the mem_section structure.
Other compaction code checks for pfn_valid(pfn), which actually checks
for the null pointer in the mem_section structure. It is not clear to
me why isolate_migratepages uses pfn_valid_within(). Changing it to
pfn_valid() prevents the crash. It looks like the correct solution to
me, but I'm not familiar with this code.
I also tried this on a 64-bit machine with a 1GB gap at 3GB, but the
address calculated from (struct page *)0 + pfn is a valid readable
memory location, so it doesn't panic. Not sure what other bad things
happen later though.
Any comments, questions, other data you'd like to see?
Thanks,
Herbert.
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