Re: [PATCH] s390: Disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Heiko Carstens <hca@linux.ibm.com> writes:

> As reported by Luiz Capitulino enabling HVO on s390 leads to reproducible
> crashes. The problem is that kernel page tables are modified without
> flushing corresponding TLB entries.
>
> Even if it looks like the empty flush_tlb_all() implementation on s390 is
> the problem, it is actually a different problem: on s390 it is not allowed
> to replace an active/valid page table entry with another valid page table
> entry without the detour over an invalid entry. A direct replacement may
> lead to random crashes and/or data corruption.
>
> In order to invalidate an entry special instructions have to be used
> (e.g. ipte or idte). Alternatively there are also special instructions
> available which allow to replace a valid entry with a different valid
> entry (e.g. crdte or cspg).
>
> Given that the HVO code currently does not provide the hooks to allow for
> an implementation which is compliant with the s390 architecture
> requirements, disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP again, which is
> basically a revert of the original patch which enabled it.
>
> Reported-by: Luiz Capitulino <luizcap@redhat.com>
> Closes: https://lore.kernel.org/all/20251028153930.37107-1-luizcap@redhat.com/
> Fixes: 00a34d5a99c0 ("s390: select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP")
> Cc: stable@vger.kernel.org
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/Kconfig | 1 -
>  1 file changed, 1 deletion(-)

Yup architectures like aarch64 and PPC64 keep it disable for the same
reason. However patch [1] allows for enabling similar optimization for
DAX vmemmap "ARCH_WANT_OPTIMIZE_DAX_VMEMMAP", if DAX is supported.

I see in 6.18-1 pull request s390 had "Add DAX support for DCSS memory
block devices".

[1]: https://lore.kernel.org/all/20230724190759.483013-8-aneesh.kumar@linux.ibm.com/T/#u

-ritesh

>
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index c4145672ca34..df22b10d9141 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -158,7 +158,6 @@ config S390
>  	select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
>  	select ARCH_WANT_KERNEL_PMD_MKWRITE
>  	select ARCH_WANT_LD_ORPHAN_WARN
> -	select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
>  	select ARCH_WANTS_THP_SWAP
>  	select BUILDTIME_TABLE_SORT
>  	select CLONE_BACKWARDS2
> -- 
> 2.48.1