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Message-ID: <1a55f9f3-f5b1-4761-97ba-423756c707fe@redhat.com>
Date: Thu, 26 Jun 2025 15:16:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org, kasong@...cent.com,
lance.yang@...ux.dev, 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 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? :)
> - 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? :)
--
Cheers,
David / dhildenb
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