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

[David Hildenbrand <david@redhat.com>]

On 25.06.25 12:47, Lance Yang wrote:
> 
> 
> On 2025/6/25 18:00, David Hildenbrand wrote:
>> On 24.06.25 18:25, Lance Yang wrote:
>>> On 2025/6/24 23:34, David Hildenbrand wrote:
>>>> On 24.06.25 17:26, Lance Yang wrote:
>>>>> On 2025/6/24 20:55, David Hildenbrand wrote:
>>>>>> On 14.02.25 10:30, Barry Song wrote:
>>>>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>> [...]
>>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>>>> index 89e51a7a9509..8786704bd466 100644
>>>>>>> --- a/mm/rmap.c
>>>>>>> +++ b/mm/rmap.c
>>>>>>> @@ -1781,6 +1781,25 @@ 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)
>>>>>>> +{
>>>>>>> +    const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
>>>>>>> FPB_IGNORE_SOFT_DIRTY;
>>>>>>> +    int max_nr = folio_nr_pages(folio);
>>>>>>
>>>>>> Let's assume we have the first page of a folio mapped at the last page
>>>>>> table entry in our page table.
>>>>>
>>>>> Good point. I'm curious if it is something we've seen in practice ;)
>>>>
>>>> I challenge you to write a reproducer :P I assume it might be doable
>>>> through simple mremap().
>>>>
>>>>>
>>>>>>
>>>>>> What prevents folio_pte_batch() from reading outside the page table?
>>>>>
>>>>> Assuming such a scenario is possible, to prevent any chance of an
>>>>> out-of-bounds read, how about this change:
>>>>>
>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>> index fb63d9256f09..9aeae811a38b 100644
>>>>> --- a/mm/rmap.c
>>>>> +++ b/mm/rmap.c
>>>>> @@ -1852,6 +1852,25 @@ static inline bool
>>>>> can_batch_unmap_folio_ptes(unsigned long addr,
>>>>>         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);
>>>>> +    unsigned long end_addr;
>>>>> +
>>>>> +    /*
>>>>> +     * To batch unmap, the entire folio's PTEs must be contiguous
>>>>> +     * and mapped within the same PTE page table, which corresponds to
>>>>> +     * a single PMD entry. Before calling folio_pte_batch(), which
>>>>> does
>>>>> +     * not perform boundary checks itself, we must verify that the
>>>>> +     * address range covered by the folio does not cross a PMD
>>>>> boundary.
>>>>> +     */
>>>>> +    end_addr = addr + (max_nr * PAGE_SIZE) - 1;
>>>>> +
>>>>> +    /*
>>>>> +     * A fast way to check for a PMD boundary cross is to align both
>>>>> +     * the start and end addresses to the PMD boundary and see if they
>>>>> +     * are different. If they are, the range spans across at least two
>>>>> +     * different PMD-managed regions.
>>>>> +     */
>>>>> +    if ((addr & PMD_MASK) != (end_addr & PMD_MASK))
>>>>> +        return false;
>>>>
>>>> You should not be messing with max_nr = folio_nr_pages(folio) here at
>>>> all. folio_pte_batch() takes care of that.
>>>>
>>>> Also, way too many comments ;)
>>>>
>>>> You may only batch within a single VMA and within a single page table.
>>>>
>>>> So simply align the addr up to the next PMD, and make sure it does not
>>>> exceed the vma end.
>>>>
>>>> ALIGN and friends can help avoiding excessive comments.
>>>
>>> Thanks! How about this updated version based on your suggestion:
>>>
>>> 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?
> 
> Yeah, that looks much better. Thanks for the suggestion!
> 
>>
>> 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);
>> +       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?
> 
> That's a good point about the "all-or-nothing" batching logic ;)
> 
> It seems the "all-or-nothing" approach is specific to the lazyfree use
> case, which needs to unmap the entire folio for reclamation. If that's
> not possible, it falls back to the single-page slow path.
> 
> Also, supporting partial batches would be useful, but not common case
> I guess, so let's leave it as is ;p

We can literally make this code less complicated if we just support it? :)

I mean, it's dropping 3 if conditions from the code I shared.

-- 
Cheers,

David / dhildenb