Aneesh Kumar K.V's on January 8, 2020 3:44 pm:
> Without this kernel can endup with SLB entries as below
>
> 04 c00c0800 00066bde000a7510 256M ESID=c00c0 VSID= 66bde000a7
> LLP:110
> 12 0800 00066bde000a7d90 256M ESID=0 VSID= 66bde000a7
> LLP:110
>
> Such SLB entries can result in machine check.
>
> We noticed this with 256MB segments because that resulted in the duplicate
> VSID
> with first vmemmap segment and first user segement. With 1TB segments we
> observe
> duplication with EAs like
>
> 0x100e64b vsid for EA 0xc00db500 context 7
> 0x100e64b vsid for user EA 0x1b500 context 7
>
> and those high addresses are not common and the kernel mapping in the above
> case
> is I/O remap range.
>
> [0.00] vmalloc start = 0xc008
> [0.00] IO start = 0xc00a
> [0.00] vmemmap start = 0xc00c
>
> Fixes: 0034d395f89d ("powerpc/mm/hash64: Map all the kernel regions in the
> same 0xc range")
> Signed-off-by: Aneesh Kumar K.V
> ---
> arch/powerpc/include/asm/book3s/64/mmu-hash.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/mmu-hash.h
> b/arch/powerpc/include/asm/book3s/64/mmu-hash.h
> index 15b75005bc34..516db8a2e6ca 100644
> --- a/arch/powerpc/include/asm/book3s/64/mmu-hash.h
> +++ b/arch/powerpc/include/asm/book3s/64/mmu-hash.h
> @@ -601,7 +601,7 @@ extern void slb_set_size(u16 size);
> */
> #define MAX_USER_CONTEXT ((ASM_CONST(1) << CONTEXT_BITS) - 2)
> #define MIN_USER_CONTEXT (MAX_KERNEL_CTX_CNT + MAX_VMALLOC_CTX_CNT + \
> - MAX_IO_CTX_CNT + MAX_VMEMMAP_CTX_CNT)
> + MAX_IO_CTX_CNT + MAX_VMEMMAP_CTX_CNT + 1)
Good find and fix, but the changelog is a bit difficult to read.
The bug is an off-by-one error which means the first user context ID
allocated is the vmemmap ID, right? I would lead with that.
I'm not sure that machine checks are a primary symptom, different ESID
mapping the same VSID is allowed. My guess is the machine check happens
a little later, after the vmemmap gets corrupted via its new mapping.
Guessing this hasn't immediately resulted in wholesale mayhem because
- Init is pretty small, doesn't use many segments or pages.
- Low 1TB is mapped with 256MB segments which get a different VA hash
than the 1TB vmemmap segment.
- init tends to load itself at 256MB, so even with disable_1tb_segments,
it's clashing with the second vmmemap segment, which is for like the
second 256GB of memory on the first node, so not going to hit many
systems.
Anyway good find.
Reviewed-by: Nicholas Piggin
Thanks,
Nick
