[RFC PATCH 1/4] powerpc/64s/radix: Fix memory hotplug section page table creation

[Nicholas Piggin]

Michael Ellerman's on July 23, 2019 8:52 pm:
> Nicholas Piggin <npiggin at gmail.com> writes:
>> create_physical_mapping expects physical addresses, but creating and
>> splitting these mappings after boot is supplying virtual (effective)
>> addresses. This can be hit by booting with limited memory then probing
>> new physical memory sections.
>>
>> Cc: Reza Arbab <arbab at linux.vnet.ibm.com>
>> Fixes: 6cc27341b21a8 ("powerpc/mm: add radix__create_section_mapping()")
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> 
> This is not catastrophic because create_physical_mapping() just uses
> start/end to construct virtual addresses anyway, and __va(__va(x)) == __va(x) ?

A bit more subtle, it calls __map_kernel_page with the pa as well.
pfn_pte ends up masking the top 0xc bits out with PTE_RPN_MASK,
which is what saves us.

Hmm, so we really should also have a VM_BUG_ON in pfn_pte if it's
given a pfn with the top PAGE_SHIFT bit or PTE_RPN_MASK bits set.
I'll add that as a patch 5.
 
> Although we do pass those through as region_start/end which then go to
> memblock_alloc_try_nid(). But I guess that doesn't happen after boot,
> which is the case you're talking about.
> 
> So I think looks good, change log could use a bit more detail though :)

Thanks for taking a look. I'll resend after a bit more testing and
some changelog improvement.

Thanks,
Nick