Re: [RESEND v7 08/29] mm: recognize PMD swap entries in the softleaf layer

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/14/26 14:27, Usama Arif wrote:
> Reclaim splits a PMD-mapped anonymous THP into PTE-level swap entries
> before unmapping it, so an ordinary swap entry has never had to appear in a
> PMD. Later patches install one there instead, and the softleaf layer is

"Prepare for ..."

> where every consumer decodes non-present PMDs.
> 
> Accept swap entries as valid PMD softleaves and add pmd_is_swap_entry().
> A swap entry carries no PFN, so make pmd_softleaf_to_folio() warn and
> return NULL rather than interpret a swap offset as a page frame number.
> 
> Unlike migration and device-private entries, a PMD swap entry can also
> carry the swap-exclusive marker, which softleaf_from_pmd() has to strip
> before decoding. Strip all three overlays unconditionally while we are
> here: each clear is a plain bit clear, so testing first only buys a branch.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---


[...]

> +/**
> + * pmd_is_swap_entry() - Does this PMD entry encode an actual swap entry?
> + * @pmd: PMD entry.
> + *
> + * Returns: true if the PMD encodes a swap entry, otherwise false.
> + */
> +static inline bool pmd_is_swap_entry(pmd_t pmd)
> +{
> +	return softleaf_is_swap(softleaf_from_pmd(pmd));
>  }

We don't have a pte_is_swap_entry(), I guess because we always want to handle
different softleaf types through softleaf_from_pte() first.

>  
>  /**
>   * pmd_is_valid_softleaf() - Is this PMD entry a valid softleaf entry?
>   * @pmd: PMD entry.
>   *
> - * PMD leaf entries are valid only if they are device private or migration
> - * entries. This function asserts that a PMD leaf entry is valid in this
> - * respect.
> + * PMD leaf entries are valid only if they are device private, migration,
> + * or swap entries. This function asserts that a PMD leaf entry is valid
> + * in this respect.
>   *
>   * Returns: true if the PMD entry is a valid leaf entry, otherwise false.
>   */
> @@ -660,10 +670,12 @@ static inline bool pmd_is_valid_softleaf(pmd_t pmd)
>   * pmd_softleaf_to_folio() - Convert the PMD softleaf entry to a folio.
>   * @pmd: PMD entry.
>   *
> - * The PMD entry is expected to be a valid PMD softleaf entry.
> + * The PMD entry is expected to be a valid PMD softleaf entry that references a
> + * PFN, that is a migration or device private entry. A PMD swap entry is a valid
> + * softleaf entry but encodes swap slots rather than a PFN, so it has no folio.
>   *
> - * Returns: the folio the softleaf entry references if this is a valid softleaf
> - * entry, otherwise NULL.
> + * Returns: the folio the softleaf entry references, or NULL if the entry is not
> + * a valid PMD softleaf entry or does not reference a PFN.
>   */
>  static inline struct folio *pmd_softleaf_to_folio(pmd_t pmd)
>  {
> @@ -673,6 +685,10 @@ static inline struct folio *pmd_softleaf_to_folio(pmd_t pmd)
>  		VM_WARN_ON_ONCE(true);
>  		return NULL;
>  	}
> +	if (!softleaf_has_pfn(entry)) {
> +		VM_WARN_ON_ONCE(true);
> +		return NULL;
softleaf_to_folio() does a

	VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));

Just curious: why is that not sufficient? Which could would we expect to call
into this fuction with nonesense? (same applies to the existing runtime check)


Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David