Re: [PATCH] mm: export folio_pte_batch as a couple of modules might need it

David Hildenbrand
2024-02-27T09:53:26Z

On 27.02.24 10:51, Lance Yang wrote:
> On Tue, Feb 27, 2024 at 5:14 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 27.02.24 10:07, Ryan Roberts wrote:
>>> On 27/02/2024 02:40, Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>
>>>> madvise and some others might need folio_pte_batch to check if a range
>>>> of PTEs are completely mapped to a large folio with contiguous physcial
>>>> addresses. Let's export it for others to use.
>>>>
>>>> Cc: Lance Yang <ioworker0@gmail.com>
>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>>> Cc: David Hildenbrand <david@redhat.com>
>>>> Cc: Yin Fengwei <fengwei.yin@intel.com>
>>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>>> ---
>>>> -v1:
>>>> at least two jobs madv_free and madv_pageout depend on it. To avoid
>>>> conflicts and dependencies, after discussing with Lance, we prefer
>>>> this one can land earlier.
>>>
>>> I think this will also ultimately be useful for mprotect too, though I haven't
>>> looked at it properly yet.
>>>
>>
>> Yes, I think we briefly discussed that.
>>
>>>>
>>>> mm/internal.h | 13 +++++++++++++
>>>> mm/memory.c | 11 +----------
>>>> 2 files changed, 14 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/mm/internal.h b/mm/internal.h
>>>> index 13b59d384845..8e2bc304f671 100644
>>>> --- a/mm/internal.h
>>>> +++ b/mm/internal.h
>>>> @@ -83,6 +83,19 @@ static inline void *folio_raw_mapping(struct folio *folio)
>>>> return (void *)(mapping & ~PAGE_MAPPING_FLAGS);
>>>> }
>>>>
>>>> +/* Flags for folio_pte_batch(). */
>>>> +typedef int __bitwise fpb_t;
>>>> +
>>>> +/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
>>>> +#define FPB_IGNORE_DIRTY ((__force fpb_t)BIT(0))
>>>> +
>>>> +/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
>>>> +#define FPB_IGNORE_SOFT_DIRTY ((__force fpb_t)BIT(1))
>>>> +
>>>> +extern int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>> + pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>>>> + bool *any_writable);
>>>> +
>>>> void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
>>>> int nr_throttled);
>>>> static inline void acct_reclaim_writeback(struct folio *folio)
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 1c45b6a42a1b..319b3be05e75 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -953,15 +953,6 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
>>>> set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
>>>> }
>>>>
>>>> -/* Flags for folio_pte_batch(). */
>>>> -typedef int __bitwise fpb_t;
>>>> -
>>>> -/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
>>>> -#define FPB_IGNORE_DIRTY ((__force fpb_t)BIT(0))
>>>> -
>>>> -/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
>>>> -#define FPB_IGNORE_SOFT_DIRTY ((__force fpb_t)BIT(1))
>>>> -
>>>> static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>>> {
>>>> if (flags & FPB_IGNORE_DIRTY)
>>>> @@ -982,7 +973,7 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>>> * If "any_writable" is set, it will indicate if any other PTE besides the
>>>> * first (given) PTE is writable.
>>>> */
>>>
>>> David was talking in Lance's patch thread, about improving the docs for this
>>> function now that its exported. Might be worth syncing on that.
>>
>> Here is my take:
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/memory.c | 22 ++++++++++++++++++----
>> 1 file changed, 18 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d0b855a1837a8..098356b8805ae 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -971,16 +971,28 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>> return pte_wrprotect(pte_mkold(pte));
>> }
>>
>> -/*
>> +/**
>> + * folio_pte_batch - detect a PTE batch for a large folio
>> + * @folio: The large folio to detect a PTE batch for.
>> + * @addr: The user virtual address the first page is mapped at.
>> + * @start_ptep: Page table pointer for the first entry.
>> + * @pte: Page table entry for the first page.
>> + * @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.
>> + *
>> * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>> - * pages of the same folio.
>> + * pages of the same large folio.
>> *
>> * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN,
>> * the accessed bit, writable bit, dirty bit (with FPB_IGNORE_DIRTY) and
>> * soft-dirty bit (with FPB_IGNORE_SOFT_DIRTY).
>> *
>> - * If "any_writable" is set, it will indicate if any other PTE besides the
>> - * first (given) PTE is writable.
>> + * start_ptep must map any page of the folio. max_nr must be at least one and
>> + * must be limited by the caller so scanning cannot exceed a single page table.
>> + *
>> + * Return: the number of table entries in the batch.
>> */
>> static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>> pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>> @@ -996,6 +1008,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>> *any_writable = 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);
>
> Nit:
> IIUC, the pte that maps to the first page.
> - VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) !=
> folio, folio);
> + VM_WARN_ON_FOLIO(pte_pfn(pte) != folio_pfn(folio), folio);

That would only work if the PTE would map the very first subpage of the
folio, not any subpage?

--
Cheers,

David / dhildenb