Re: [PATCH 1/2] KVM: Set up async page faults only for faults that need to wait

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 20, 2026, Christian Borntraeger wrote:
> hva_to_pfn() checks a FOLL_NOWAIT gup() for EINTR, EAGAIN and EHWPOISON.
> There might be others like -EFAULT for VM_FAULT_SIGBUS.  For those other
> errors, reporting KVM_PFN_ERR_NEEDS_IO would be wrong as it triggers an
> async_pf resulting in endless retries.
> 
> Return KVM_PFN_ERR_NEEDS_IO only if gup() returned 0 pages and report
> the error otherwise.
> 
> Fixes: 0857b9e95c1a ("KVM: Enable async page fault processing")
> Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> ---
>  virt/kvm/kvm_main.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5a782849521c..34b38d7512c3 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3048,6 +3048,7 @@ kvm_pfn_t hva_to_pfn(struct kvm_follow_pfn *kfp)
>  			pfn = KVM_PFN_ERR_FAULT;
>  	} else {
>  		if ((kfp->flags & FOLL_NOWAIT) &&
> +		    npages == 0 && /* no PFN_ERR_NEEDS_IO on errors */

!npages is generally preferred over "== 0".


>  		    vma_is_valid(vma, kfp->flags & FOLL_WRITE))
>  			pfn = KVM_PFN_ERR_NEEDS_IO;
>  		else

Maybe instead of calling out "on errors", explicitly call out that GUP returns '0'
when the fault needs to be retried?  And if we move the npages check first, then
IMO we can use a block comment instead of a trailing comment, e.g.

		/* Note, GUP returns '0' if retry is needed. */
		if (!npages && (kfp->flags & FOLL_NOWAIT) &&
		    vma_is_valid(vma, kfp->flags & FOLL_WRITE))
			pfn = KVM_PFN_ERR_NEEDS_IO;
		else
			pfn = KVM_PFN_ERR_FAULT;


For anyone else that found this surprising, while faultin_page() returns -EBUSY
or -EAGAIN, __get_user_pages() converts those to '0':

			ret = faultin_page(vma, start, gup_flags,
					   PTR_ERR(page) == -EMLINK, locked);
			switch (ret) {
			case 0:
				goto retry;
			case -EBUSY:
			case -EAGAIN:
				ret = 0;
				fallthrough;
			case -EFAULT:
			case -ENOMEM:
			case -EHWPOISON:
				goto out;
			}

It'd be loved if GUP return EWOULDBLOCK for this case, but I can only imagine the
pain that could cause for the rest of the kernel.