Re: [PATCH v1 1/2] mm/userfaultfd: don't place zeropages when zeropages are disallowed

[Peter Xu <peterx@redhat.com>]

On Thu, Mar 21, 2024 at 10:59:53PM +0100, David Hildenbrand wrote:
> s390x must disable shared zeropages for processes running VMs, because
> the VMs could end up making use of "storage keys" or protected
> virtualization, which are incompatible with shared zeropages.
> 
> Yet, with userfaultfd it is possible to insert shared zeropages into
> such processes. Let's fallback to simply allocating a fresh zeroed
> anonymous folio and insert that instead.
> 
> mm_forbids_zeropage() was introduced in commit 593befa6ab74 ("mm: introduce
> mm_forbids_zeropage function"), briefly before userfaultfd went
> upstream.
> 
> Note that we don't want to fail the UFFDIO_ZEROPAGE request like we do
> for hugetlb, it would be rather unexpected. Further, we also
> cannot really indicated "not supported" to user space ahead of time: it
> could be that the MM disallows zeropages after userfaultfd was already
> registered.
> 
> Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

Still, a few comments below.

> ---
>  mm/userfaultfd.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 712160cd41eca..1d1061ccd1dea 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -316,6 +316,38 @@ static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
>  	goto out;
>  }
>  
> +static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
> +		 struct vm_area_struct *dst_vma, unsigned long dst_addr)
> +{
> +	struct folio *folio;
> +	int ret;

nitpick: we can set -ENOMEM here, then

> +
> +	folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
> +	if (!folio)
> +		return -ENOMEM;

return ret;

> +
> +	ret = -ENOMEM;

drop.

> +	if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
> +		goto out_put;
> +
> +	/*
> +	 * The memory barrier inside __folio_mark_uptodate makes sure that
> +	 * preceding stores to the page contents become visible before
> +	 * the set_pte_at() write.
> +	 */

This comment doesn't apply.  We can drop it.

Thanks,

> +	__folio_mark_uptodate(folio);
> +
> +	ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
> +				       &folio->page, true, 0);
> +	if (ret)
> +		goto out_put;
> +
> +	return 0;
> +out_put:
> +	folio_put(folio);
> +	return ret;
> +}
> +
>  static int mfill_atomic_pte_zeropage(pmd_t *dst_pmd,
>  				     struct vm_area_struct *dst_vma,
>  				     unsigned long dst_addr)
> @@ -324,6 +356,9 @@ static int mfill_atomic_pte_zeropage(pmd_t *dst_pmd,
>  	spinlock_t *ptl;
>  	int ret;
>  
> +	if (mm_forbids_zeropage(dst_vma->mm))
> +		return mfill_atomic_pte_zeroed_folio(dst_pmd, dst_vma, dst_addr);
> +
>  	_dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
>  					 dst_vma->vm_page_prot));
>  	ret = -EAGAIN;
> -- 
> 2.43.2
> 

-- 
Peter Xu