Re: [PATCH v1 4/4] mm: remove boolean output parameters from folio_pte_batch_ext()

[David Hildenbrand <david@redhat.com>]

On 30.06.25 19:59, Zi Yan wrote:
> On 27 Jun 2025, at 7:55, David Hildenbrand wrote:
> 
>> Instead, let's just allow for specifying through flags whether we want
>> to have bits merged into the original PTE.
>>
>> For the madvise() case, simplify by having only a single parameter for
>> merging young+dirty. For madvise_cold_or_pageout_pte_range() merging the
>> dirty bit is not required, but also not harmful. This code is not that
>> performance critical after all to really force all micro-optimizations.
>>
>> As we now have two pte_t * parameters, use PageTable() to make sure we
>> are actually given a pointer at a copy of the PTE, not a pointer into
>> an actual page table.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   mm/internal.h | 58 +++++++++++++++++++++++++++++++--------------------
>>   mm/madvise.c  | 26 +++++------------------
>>   mm/memory.c   |  8 ++-----
>>   mm/util.c     |  2 +-
>>   4 files changed, 43 insertions(+), 51 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 6000b683f68ee..fe69e21b34a24 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -208,6 +208,18 @@ typedef int __bitwise fpb_t;
>>   /* Compare PTEs honoring the soft-dirty bit. */
>>   #define FPB_HONOR_SOFT_DIRTY		((__force fpb_t)BIT(1))
>>
>> +/*
>> + * Merge PTE write bits: if any PTE in the batch is writable, modify the
>> + * PTE at @ptentp to be writable.
>> + */
>> +#define FPB_MERGE_WRITE			((__force fpb_t)BIT(2))
>> +
>> +/*
>> + * Merge PTE young and dirty bits: if any PTE in the batch is young or dirty,
>> + * modify the PTE at @ptentp to be young or dirty, respectively.
>> + */
>> +#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(3))
>> +
>>   static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>   {
>>   	if (!(flags & FPB_HONOR_DIRTY))
>> @@ -220,16 +232,11 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>   /**
>>    * folio_pte_batch_ext - detect a PTE batch for a large folio
>>    * @folio: The large folio to detect a PTE batch for.
>> + * @vma: The VMA. Only relevant with FPB_MERGE_WRITE, otherwise can be NULL.
>>    * @ptep: Page table pointer for the first entry.
>> - * @pte: Page table entry for the first page.
>> + * @ptentp: Pointer at a copy of the first page table entry.
>>    * @max_nr: The maximum number of table entries to consider.
>>    * @flags: Flags to modify the PTE batch semantics.
>> - * @any_writable: Optional pointer to indicate whether any entry except the
>> - *		  first one is writable.
>> - * @any_young: Optional pointer to indicate whether any entry except the
>> - *		  first one is young.
>> - * @any_dirty: Optional pointer to indicate whether any entry except the
>> - *		  first one is dirty.
>>    *
>>    * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>>    * pages of the same large folio in a single VMA and a single page table.
>> @@ -242,28 +249,26 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>    * must be limited by the caller so scanning cannot exceed a single VMA and
>>    * a single page table.
>>    *
>> + * Depending on the FPB_MERGE_* flags, the pte stored at @ptentp will
>> + * be modified.
>> + *
>>    * This function will be inlined to optimize based on the input parameters;
>>    * consider using folio_pte_batch() instead if applicable.
>>    *
>>    * Return: the number of table entries in the batch.
>>    */
>>   static inline unsigned int folio_pte_batch_ext(struct folio *folio,
>> -		pte_t *ptep, pte_t pte, unsigned int max_nr, fpb_t flags,
>> -		bool *any_writable, bool *any_young, bool *any_dirty)
>> +		struct vm_area_struct *vma, pte_t *ptep, pte_t *ptentp,
>> +		unsigned int max_nr, fpb_t flags)
>>   {
>> +	bool any_writable = false, any_young = false, any_dirty = false;
>> +	pte_t expected_pte, pte = *ptentp;
>>   	unsigned int nr, cur_nr;
>> -	pte_t expected_pte;
>> -
>> -	if (any_writable)
>> -		*any_writable = false;
>> -	if (any_young)
>> -		*any_young = false;
>> -	if (any_dirty)
>> -		*any_dirty = false;
>>
>>   	VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>>   	VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
>>   	VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);
>> +	VM_WARN_ON(virt_addr_valid(ptentp) && PageTable(virt_to_page(ptentp)));
> 
> Why not just VM_WARN_ON(!pte_same(*ptep, *ptentp)) ?
> 
> ptep points to the first page table entry and ptentp points to the copy of it.
> I assume ptep should point to a valid page table entry to begin with.

That would also work, only miss the case where someone would by accident 
flip both pointers :)

-- 
Cheers,

David / dhildenb