Re: [PATCH] s390: Disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP

[Gerald Schaefer]

On Thu, 30 Oct 2025 15:55:05 +0100
Heiko Carstens <hca@xxxxxxxxxxxxx> wrote:

> 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@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20251028153930.37107-1-luizcap@xxxxxxxxxx/
> Fixes: 00a34d5a99c0 ("s390: select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
> ---
>  arch/s390/Kconfig | 1 -
>  1 file changed, 1 deletion(-)

Thanks!

Reviewed-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>