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Message-ID: <146b4cb1-aa1e-4519-9e03-f98cfb1135d2@redhat.com>
Date: Wed, 25 Jun 2025 12:59:20 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org, kasong@...cent.com,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
lorenzo.stoakes@...cle.com, ryan.roberts@....com, v-songbaohua@...o.com,
x86@...nel.org, ying.huang@...el.com, zhengtangquan@...o.com
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
folios during reclamation
On 25.06.25 12:49, Barry Song wrote:
> On Wed, Jun 25, 2025 at 10:43 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> 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.
>
> It's not necessarily more complex, since page_vma_mapped_walk() still
> has to check each PTE individually and can't skip ahead based on nr.
> With nr_pages batched, we can exit the loop early in one go.
I said "complicated", not "complex". The code is more complicated than
necessary.
--
Cheers,
David / dhildenb
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