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

[Sachin Sant]

> On 30-Apr-2019, at 1:29 PM, Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com> wrote:
> 
> 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>
> —

Thanks for the patch. Fixes the kernel crash.

Tested-by: Sachin Sant <sachinp at linux.vnet.ibm.com <mailto:sachinp at linux.vnet.ibm.com>>

Thanks
-Sachin
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