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

[David Hildenbrand <david@redhat.com>]

On 26.06.25 15:52, Lance Yang wrote:
> 
> 
> On 2025/6/26 21:16, David Hildenbrand wrote:
>> On 26.06.25 14:44, Lance Yang wrote:
>>>
>>> On 2025/6/26 17:29, Lance Yang wrote:
>>>> Before I send out the real patch, I'd like to get some quick feedback to
>>>> ensure I've understood the discussion correctly ;)
>>>>
>>>> Does this look like the right direction?
>>>>
>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>> index fb63d9256f09..5ebffe2137e4 100644
>>>> --- a/mm/rmap.c
>>>> +++ b/mm/rmap.c
>>>> @@ -1845,23 +1845,37 @@ 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);
>>>> +    unsigned long end_addr, addr = pvmw->address;
>>>> +    struct vm_area_struct *vma = pvmw->vma;
>>>> +    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 = pmd_addr_end(addr, 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;
>>>> +    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
>>>>     }
>>>>     /*
>>>> @@ -2024,9 +2038,7 @@ 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);
>>>> @@ -2206,13 +2218,16 @@ static bool try_to_unmap_one(struct folio
>>>> *folio, struct vm_area_struct *vma,
>>>>                 hugetlb_remove_rmap(folio);
>>>>             } else {
>>>>                 folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
>>>> -            folio_ref_sub(folio, nr_pages - 1);
>>>>             }
>>>>             if (vma->vm_flags & VM_LOCKED)
>>>>                 mlock_drain_local();
>>>> -        folio_put(folio);
>>>> -        /* We have already batched the entire folio */
>>>> -        if (nr_pages > 1)
>>>> +        folio_put_refs(folio, nr_pages);
>>>> +
>>>> +        /*
>>>> +         * If we are sure that we batched the entire folio and cleared
>>>> +         * all PTEs, we can just optimize and stop right here.
>>>> +         */
>>>> +        if (nr_pages == folio_nr_pages(folio))
>>>>                 goto walk_done;
>>>>             continue;
>>>>     walk_abort:
>>>> -- 
>>>
>>> Oops ... Through testing on my machine, I found that the logic doesn't
>>> behave as expected because I messed up the meaning of max_nr (the
>>> available
>>> scan room in the page table) with folio_nr_pages(folio) :(
>>>
>>> With the following change:
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 5ebffe2137e4..b1407348e14e 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1850,9 +1850,9 @@ static inline unsigned int
>>> folio_unmap_pte_batch(struct folio *folio,
>>>                enum ttu_flags flags, pte_t pte)
>>>    {
>>>        const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>> +    unsigned int max_nr, nr_pages = folio_nr_pages(folio);
>>>        unsigned long end_addr, addr = pvmw->address;
>>>        struct vm_area_struct *vma = pvmw->vma;
>>> -    unsigned int max_nr;
>>>        if (flags & TTU_HWPOISON)
>>>            return 1;
>>> @@ -1870,12 +1870,13 @@ static inline unsigned int
>>> folio_unmap_pte_batch(struct folio *folio,
>>>            return 1;
>>>        /* ... where we must be able to batch the whole folio. */
>>
>> Why is that still required? :)
> 
> Sorry ... I was still stuck in the "all-or-nothing" mindset ...
> 
> So, IIUC, you mean we should completely remove the "max_nr < nr_pages"
> check and just let folio_pte_batch handle whatever partial batch it
> safely can.
> 
>>
>>> -    if (pte_pfn(pte) != folio_pfn(folio) || max_nr !=
>>> folio_nr_pages(folio))
>>> +    if (pte_pfn(pte) != folio_pfn(folio) || max_nr < nr_pages)
>>>            return 1;
>>> -    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr,
>>> fpb_flags,
>>> -                 NULL, NULL, NULL);
>>> -    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
>>> +    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, nr_pages,
>>> +                 fpb_flags, NULL, NULL, NULL);
>>> +
>>> +    return (max_nr != nr_pages) ? 1 : max_nr;
>>
>> Why is that still required? :)
> 
> Then simply return the number of PTEs that consecutively map to the
> large folio. Right?

Yes. Any part of the large folio. Just return folio_pte_batch() ...

-- 
Cheers,

David / dhildenb