Re: [PATCH] powerpc/mm: Fix crashes with hugepages & 4K pages

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Michael Ellerman <mpe@ellerman.id.au> writes:

> The recent commit to cleanup ifdefs in the hugepage initialisation led
> to crashes when using 4K pages as reported by Sachin:
>
>   BUG: Kernel NULL pointer dereference at 0x0000001c
>   Faulting instruction address: 0xc000000001d1e58c
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   LE PAGE_SIZE=4K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
>   ...
>   CPU: 3 PID: 4635 Comm: futex_wake04 Tainted: G        W  O      5.1.0-next-20190507-autotest #1
>   NIP:  c000000001d1e58c LR: c000000001d1e54c CTR: 0000000000000000
>   REGS: c000000004937890 TRAP: 0300
>   MSR:  8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 22424822  XER: 00000000
>   CFAR: c00000000183e9e0 DAR: 000000000000001c DSISR: 40000000 IRQMASK: 0
>   ...
>   NIP kmem_cache_alloc+0xbc/0x5a0
>   LR  kmem_cache_alloc+0x7c/0x5a0
>   Call Trace:
>     huge_pte_alloc+0x580/0x950
>     hugetlb_fault+0x9a0/0x1250
>     handle_mm_fault+0x490/0x4a0
>     __do_page_fault+0x77c/0x1f00
>     do_page_fault+0x28/0x50
>     handle_page_fault+0x18/0x38
>
> This is caused by us trying to allocate from a NULL kmem cache in
> __hugepte_alloc(). The kmem cache is NULL because it was never
> allocated in hugetlbpage_init(), because add_huge_page_size() returned
> an error.
>
> The reason add_huge_page_size() returned an error is a simple typo, we
> are calling check_and_get_huge_psize(size) when we should be passing
> shift instead.
>
> The fact that we're able to trigger this path when the kmem caches are
> NULL is a separate bug, ie. we should not advertise any hugepage sizes
> if we haven't setup the required caches for them.
>
> This was only seen with 4K pages, with 64K pages we don't need to
> allocate any extra kmem caches because the 16M hugepage just occupies
> a single entry at the PMD level.
>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

> Fixes: 723f268f19da ("powerpc/mm: cleanup ifdef mess in add_huge_page_size()")
> Reported-by: Sachin Sant <sachinp@linux.ibm.com>
> Tested-by: Sachin Sant <sachinp@linux.ibm.com>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/mm/hugetlbpage.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index c5c9ff2d7afc..b5d92dc32844 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -556,7 +556,7 @@ static int __init add_huge_page_size(unsigned long long size)
>  	if (size <= PAGE_SIZE || !is_power_of_2(size))
>  		return -EINVAL;
>  
> -	mmu_psize = check_and_get_huge_psize(size);
> +	mmu_psize = check_and_get_huge_psize(shift);
>  	if (mmu_psize < 0)
>  		return -EINVAL;
>  
> -- 
> 2.20.1