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

[David Hildenbrand (Arm)]

On 9/22/26 15:08, Usama Arif wrote:
> 
> 
> On 18/09/2026 22:56, David Hildenbrand (Arm) wrote:
>> 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@xxxxxxxxx>
>>> ---
>>
>>
>> [...]
>>
>>> +/**
>>> + * 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)
>>
> 
> Because softleaf_to_folio() warns after the damage:
> 
>   static inline struct folio *softleaf_to_folio(softleaf_t entry)
>   {
>           struct folio *folio = pfn_folio(softleaf_to_pfn(entry));
> 
>           VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));
> 
> pfn_folio() is page_folio(pfn_to_page(pfn)), and page_folio() is
> _compound_head(), i.e. READ_ONCE(page->compound_info).  So a swap
> offset interpreted as a PFN is already dereferenced by the time the
> warning runs.


Heh, and in softleaf_to_pfn() we have another

	VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));

So the one in softleaf_to_page() is rather useless already? :)

-- 
Cheers,

David