Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

[David Hildenbrand <david@redhat.com>]

On 25.06.25 12:38, Barry Song wrote:
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index fb63d9256f09..241d55a92a47 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1847,12 +1847,25 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
>>>
>>>    /* We support batch unmapping of PTEs for lazyfree large folios */
>>>    static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
>>> -                     struct folio *folio, pte_t *ptep)
>>> +                                           struct folio *folio, pte_t *ptep,
>>> +                                           struct vm_area_struct *vma)
>>>    {
>>>        const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>> +     unsigned long next_pmd, vma_end, end_addr;
>>>        int max_nr = folio_nr_pages(folio);
>>>        pte_t pte = ptep_get(ptep);
>>>
>>> +     /*
>>> +      * Limit the batch scan within a single VMA and within a single
>>> +      * page table.
>>> +      */
>>> +     vma_end = vma->vm_end;
>>> +     next_pmd = ALIGN(addr + 1, PMD_SIZE);
>>> +     end_addr = addr + (unsigned long)max_nr * PAGE_SIZE;
>>> +
>>> +     if (end_addr > min(next_pmd, vma_end))
>>> +             return false;
>>
>> May I suggest that we clean all that up as we fix it?
>>
>> Maybe something like this:
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 3b74bb19c11dd..11fbddc6ad8d6 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1845,23 +1845,38 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
>>    #endif
>>    }
>>
>> -/* We support batch unmapping of PTEs for lazyfree large folios */
>> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
>> -                       struct folio *folio, pte_t *ptep)
>> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
>> +               struct page_vma_mapped_walk *pvmw, enum ttu_flags flags,
>> +               pte_t pte)
>>    {
>>           const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> -       int max_nr = folio_nr_pages(folio);
>> -       pte_t pte = ptep_get(ptep);
>> +       struct vm_area_struct *vma = pvmw->vma;
>> +       unsigned long end_addr, addr = pvmw->address;
>> +       unsigned int max_nr;
>> +
>> +       if (flags & TTU_HWPOISON)
>> +               return 1;
>> +       if (!folio_test_large(folio))
>> +               return 1;
>> +
>> +       /* We may only batch within a single VMA and a single page table. */
>> +       end_addr = min_t(unsigned long, ALIGN(addr + 1, PMD_SIZE), vma->vm_end);
> 
> Is this pmd_addr_end()?
> 

Yes, that could be reused as well here.

>> +       max_nr = (end_addr - addr) >> PAGE_SHIFT;
>>
>> +       /* We only support lazyfree batching for now ... */
>>           if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
>> -               return false;
>> +               return 1;
>>           if (pte_unused(pte))
>> -               return false;
>> -       if (pte_pfn(pte) != folio_pfn(folio))
>> -               return false;
>> +               return 1;
>> +       /* ... where we must be able to batch the whole folio. */
>> +       if (pte_pfn(pte) != folio_pfn(folio) || max_nr != folio_nr_pages(folio))
>> +               return 1;
>> +       max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags,
>> +                                NULL, NULL, NULL);
>>
>> -       return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
>> -                              NULL, NULL) == max_nr;
>> +       if (max_nr != folio_nr_pages(folio))
>> +               return 1;
>> +       return max_nr;
>>    }
>>
>>    /*
>> @@ -2024,9 +2039,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>                           if (pte_dirty(pteval))
>>                                   folio_mark_dirty(folio);
>>                   } else if (likely(pte_present(pteval))) {
>> -                       if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
>> -                           can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
>> -                               nr_pages = folio_nr_pages(folio);
>> +                       nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
>>                           end_addr = address + nr_pages * PAGE_SIZE;
>>                           flush_cache_range(vma, address, end_addr);
>>
>>
>> Note that I don't quite understand why we have to batch the whole thing or fallback to
>> individual pages. Why can't we perform other batches that span only some PTEs? What's special
>> about 1 PTE vs. 2 PTEs vs. all PTEs?
>>
>>
>> Can someone enlighten me why that is required?
> 
> It's probably not a strict requirement — I thought cases where the
> count is greater than 1 but less than nr_pages might not provide much
> practical benefit, except perhaps in very rare edge cases, since
> madv_free() already calls split_folio().

Okay, but it makes the code more complicated. If there is no reason to
prevent the batching, we should drop it.

> 
> if (folio_test_large(folio)) {
>                          bool any_young, any_dirty;
>                          nr = madvise_folio_pte_batch(addr, end, folio, pte,
>                                                       ptent,
> &any_young, &any_dirty);
> 
> 
>                          if (nr < folio_nr_pages(folio)) {
>                                  ...
>                                  err = split_folio(folio);
>                                  ...
>                         }
> }
> 
> Another reason is that when we extend this to non-lazyfree anonymous
> folios [1], things get complicated: checking anon_exclusive and updating
> folio_try_share_anon_rmap_pte with the number of PTEs becomes tricky if
> a folio is partially exclusive and partially shared.

Right, but that's just another limitation on top how much we can batch, 
right?


-- 
Cheers,

David / dhildenb


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