Re: [PATCH] mm/memory: Fix missing pte marker for !page on pte zaps

[David Hildenbrand <david@redhat.com>]

On 13.03.24 22:31, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> Commit 0cf18e839f64 of large folio zap work broke uffd-wp.  Now mm's uffd
> unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().

Good that I added the WARN_ON_ONCE() :)

> 
> The WARN_ON_ONCE() asserts that an VMA cannot be registered with
> userfaultfd-wp if it contains a !normal page, but it's actually possible.
> One example is an anonymous vma, register with uffd-wp, read anything will
> install a zero page.  Then when zap on it, this should trigger.

Are you sure? zap_install_uffd_wp_if_needed() contains right at the start:

	/* Zap on anonymous always means dropping everything */
	if (vma_is_anonymous(vma))
		return;

So if that's the case the unit test triggers, I'm confused.

> 
> What's more, removing that WARN_ON_ONCE may not be enough either, because
> we should also not rely on "whether it's a normal page" to decide whether
> pte marker is needed.  For example, one can register wr-protect over some
> DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which
> case it can have page==NULL for a devmap but we may want to keep the marker
> around.

I thought uffd-wp was limited to specific backends only. But looks like 
that changed with UFFD_FEATURE_WP_ASYNC, I guess?


Change itself looks, good. Not sure about the anon_vma example above.

Thanks!

Acked-by: David Hildenbrand <david@redhat.com>

> 
> Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
> Cc: David Hildenbrand <david@redhat.com>
> Fixes: 0cf18e839f64 ("mm/memory: handle !page case in zap_present_pte() separately")
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   mm/memory.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index f2bc6dd15eb8..904f70b99498 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1536,7 +1536,9 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
>   		ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>   		arch_check_zapped_pte(vma, ptent);
>   		tlb_remove_tlb_entry(tlb, pte, addr);
> -		VM_WARN_ON_ONCE(userfaultfd_wp(vma));
> +		if (userfaultfd_pte_wp(vma, ptent))
> +			zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
> +						      details, ptent);
>   		ksm_might_unmap_zero_page(mm, ptent);
>   		return 1;
>   	}

-- 
Cheers,

David / dhildenb