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Message-ID: <6fbcf806-eb3c-4bcd-8daf-8d87fd759d2b@redhat.com>
Date: Thu, 26 Jun 2025 16:39:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: 21cnbao@...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,
Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
folios during reclamation
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
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