Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and !skeys KVM guests

Alexander Gordeev
2024-04-15T18:24:37Z

On Thu, Apr 11, 2024 at 06:14:41PM +0200, David Hildenbrand wrote:

David, could you please clarify the below questions?

> +static int __s390_unshare_zeropages(struct mm_struct *mm)
> +{
> + struct vm_area_struct *vma;
> + VMA_ITERATOR(vmi, mm, 0);
> + unsigned long addr;
> + int rc;
> +
> + for_each_vma(vmi, vma) {
> + /*
> + * We could only look at COW mappings, but it's more future
> + * proof to catch unexpected zeropages in other mappings and
> + * fail.
> + */
> + if ((vma->vm_flags & VM_PFNMAP) || is_vm_hugetlb_page(vma))
> + continue;
> + addr = vma->vm_start;
> +
> +retry:
> + rc = walk_page_range_vma(vma, addr, vma->vm_end,
> + &find_zeropage_ops, &addr);
> + if (rc <= 0)
> + continue;

So in case an error is returned for the last vma, __s390_unshare_zeropage()
finishes with that error. By contrast, the error for a non-last vma would
be ignored?

> +
> + /* addr was updated by find_zeropage_pte_entry() */
> + rc = handle_mm_fault(vma, addr,
> + FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
> + NULL);
> + if (rc & VM_FAULT_OOM)
> + return -ENOMEM;

Heiko pointed out that rc type is inconsistent vs vm_fault_t returned by
handle_mm_fault(). While fixing it up, I've got concerned whether is it
fine to continue in case any other error is met (including possible future
VM_FAULT_xxxx)?

> + /*
> + * See break_ksm(): even after handle_mm_fault() returned 0, we
> + * must start the lookup from the current address, because
> + * handle_mm_fault() may back out if there's any difficulty.
> + *
> + * VM_FAULT_SIGBUS and VM_FAULT_SIGSEGV are unexpected but
> + * maybe they could trigger in the future on concurrent
> + * truncation. In that case, the shared zeropage would be gone
> + * and we can simply retry and make progress.
> + */
> + cond_resched();
> + goto retry;
> + }
> +
> + return rc;
> +}

Thanks!