Re: [PATCH/RFC] mm: do not drop unused pages when userfaultd is running

[Christian Borntraeger]

On 06/28/2018 02:39 PM, Christian Borntraeger wrote:
> KVM guests on s390 can notify the host of unused pages. This can result
> in pte_unused callbacks to be true for KVM guest memory.
> 
> If a page is unused (checked with pte_unused) we might drop this page
> instead of paging it. This can have side-effects on userfaultd, when the
> page in question was already migrated:
> 
> The next access of that page will trigger a fault and a user fault
> instead of faulting in a new and empty zero page. As QEMU does not
> expect a userfault on an already migrated page this migration will fail.
> 
> The most straightforward solution is to ignore the pte_unused hint if a
> userfault context is active for this VMA.
> 
> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
>  mm/rmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 6db729dc4c50..3f3a72aa99f2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1481,7 +1481,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  			}
>  
> -		} else if (pte_unused(pteval)) {
> +		} else if (pte_unused(pteval) && !vma->vm_userfaultfd_ctx.ctx) {

FWIW, this needs a fix for !CONFIG_USERFAULTFD.
Still: more opinions on the patch itself?

>  			/*
>  			 * The guest indicated that the page content is of no
>  			 * interest anymore. Simply discard the pte, vmscan
>