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Message-ID: <ce78181f-b8f0-4710-be22-eff123760a51@linux.dev>
Date: Thu, 26 Jun 2025 21:52:58 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
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 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?
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