Re: [RFC PATCH v1 2/4] mm/gup: Use ptep_get_lockless_norecency()

[David Hildenbrand <david@redhat.com>]

On 15.02.24 13:17, Ryan Roberts wrote:
> Gup needs to read ptes locklessly, so it uses ptep_get_lockless().
> However, the returned access and dirty bits are unimportant so let's
> switch over to ptep_get_lockless_norecency().
> 
> The wrinkle is that gup needs to check that the pte hasn't changed once
> it has pinned the folio following this model:
> 
>      pte = ptep_get_lockless_norecency(ptep)
>      ...
>      if (!pte_same(pte, ptep_get_lockless(ptep)))
>              // RACE!
>      ...
> 
> And now that pte may not contain correct access and dirty information,
> the pte_same() comparison could spuriously fail. So let's introduce a
> new pte_same_norecency() helper which will ignore the access and dirty
> bits when doing the comparison.
> 
> Note that previously, ptep_get() was being used for the comparison; this
> is technically incorrect because the PTL is not held. I've also
> converted the comparison to use the preferred pmd_same() helper instead
> of doing a raw value comparison.
> 
> As a side-effect, this new approach removes the possibility of
> concurrent read/write to the page causing a spurious fast gup failure,
> because the access and dirty bits are no longer used in the comparison.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---

[...]

>   #ifndef __HAVE_ARCH_PTE_UNUSED
>   /*
>    * Some architectures provide facilities to virtualization guests
> diff --git a/mm/gup.c b/mm/gup.c
> index df83182ec72d..0f96d0a5ec09 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2576,7 +2576,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
>   	if (!ptep)
>   		return 0;
>   	do {
> -		pte_t pte = ptep_get_lockless(ptep);
> +		pte_t pte = ptep_get_lockless_norecency(ptep);
>   		struct page *page;
>   		struct folio *folio;
> 
> @@ -2617,8 +2617,9 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
>   			goto pte_unmap;
>   		}
> 
> -		if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
> -		    unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
> +		if (unlikely(!pmd_same(pmd, *pmdp)) ||
> +		    unlikely(!pte_same_norecency(pte,
> +					ptep_get_lockless_norecency(ptep)))) {
>   			gup_put_folio(folio, 1, flags);
>   			goto pte_unmap;

We pass the pte into pte_access_permitted(). It would be good to mention 
that you checked all implementations.

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

-- 
Cheers,

David / dhildenb