Re: [PATCH v2 2/4] mm: Add poison error check in fixup_user_fault() for mapped pfn

[Sean Christopherson]

On Thu, Nov 23, 2023, ankita@xxxxxxxxxx wrote:
> From: Ankit Agrawal <ankita@xxxxxxxxxx>
> 
> The fixup_user_fault() currently does not expect a VM_FAULT_HWPOISON
> and hence does not check for it while calling vm_fault_to_errno(). Since
> we now have a new code path which can trigger such case, change
> fixup_user_fault to look for VM_FAULT_HWPOISON.
> 
> Also make hva_to_pfn_remapped check for -EHWPOISON and communicate the
> poison fault up to the user_mem_abort().

I would much prefer the KVM change be split out to its own patch, I see no
reason why it needs to be bundled with the fixup_user_fault() change.  KVM will
set pfn to KVM_PFN_ERR_FAULT before and after the fixup_user_fault() change.

> Signed-off-by: Ankit Agrawal <ankita@xxxxxxxxxx>
> ---
>  mm/gup.c            | 2 +-
>  virt/kvm/kvm_main.c | 6 ++++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 231711efa390..b78af20a0f52 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1414,7 +1414,7 @@ int fixup_user_fault(struct mm_struct *mm,
>  	}
>  
>  	if (ret & VM_FAULT_ERROR) {
> -		int err = vm_fault_to_errno(ret, 0);
> +		int err = vm_fault_to_errno(ret, FOLL_HWPOISON);
>  
>  		if (err)
>  			return err;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 486800a7024b..2ff067f21a7c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2731,6 +2731,12 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool interruptible,
>  		r = hva_to_pfn_remapped(vma, addr, write_fault, writable, &pfn);
>  		if (r == -EAGAIN)
>  			goto retry;
> +
> +		if (r == -EHWPOISON) {
> +			pfn = KVM_PFN_ERR_HWPOISON;
> +			goto exit;
> +		}
> +
>  		if (r < 0)
>  			pfn = KVM_PFN_ERR_FAULT;

I vote for

		if (r == -EHWPOISON)
			pfn = KVM_PFN_ERR_HWPOISON;
		else if (r < 0)
			pfn = KVM_PFN_ERR_FAULT;

or even opportunstically fix the < 0 weirdness:

		if (r == -EHWPOISON)
			pfn = KVM_PFN_ERR_HWPOISON;
		else if (r)
			pfn = KVM_PFN_ERR_FAULT;

It's rather confusing to see a goto in one error path but an effective fallthrough
in a different error path, i.e. gives the impression that KVM_PFN_ERR_HWPOISON
has some special behavior that doesn't apply to KVM_PFN_ERR_FAULT.