[PATCH] powerpc/mm/radix: Fix kernel crash when running subpage protect test

[Michael Ellerman]

"Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com> writes:

> This patch fixes the below crash by making sure we touch the subpage protection
> related structures only if we know they are allocated on the platform. With
> radix translation we don't allocate hash context at all and trying to access
> subpage_prot_table results in
>
>  Faulting instruction address: 0xc00000000008bdb4
>  Oops: Kernel access of bad area, sig: 11 [#1]
>  LE PAGE_SIZE=64K MMU=Radix MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
>  ....
>  NIP [c00000000008bdb4] sys_subpage_prot+0x74/0x590
>  LR [c00000000000b688] system_call+0x5c/0x70
>  Call Trace:
>  [c00020002c6b7d30] [c00020002c6b7d90] 0xc00020002c6b7d90 (unreliable)
>  [c00020002c6b7e20] [c00000000000b688] system_call+0x5c/0x70
>  Instruction dump:
>  fb61ffd8 fb81ffe0 fba1ffe8 fbc1fff0 fbe1fff8 f821ff11 e92d1178 f9210068
>  39200000 e92d0968 ebe90630 e93f03e8 <eb891038> 60000000 3860fffe e9410068
>
> We also move the subpage_prot_table with mmp_sem held to avoid racec
> between two parallel subpage_prot syscall.
>
> Reported-by: Sachin Sant <sachinp at linux.vnet.ibm.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>

Presumably it was:

701101865f5d ("powerpc/mm: Reduce memory usage for mm_context_t for radix")

That caused the breakage?

cheers