linux-kernel - Re: [PATCH v5 5/5] mm: rmap: support batched unmapping for file large folios

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Message-ID: <CAGsJ_4yKRtyADU2ZxUMgvqdrhAEL4SpZxCMRt28gNJfUf--5zQ@mail.gmail.com>
Date: Wed, 7 Jan 2026 10:29:25 +1300
From: Barry Song <21cnbao@...il.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org, 
	david@...nel.org, catalin.marinas@....com, will@...nel.org, 
	lorenzo.stoakes@...cle.com, ryan.roberts@....com, Liam.Howlett@...cle.com, 
	vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, 
	riel@...riel.com, harry.yoo@...cle.com, jannh@...gle.com, willy@...radead.org, 
	dev.jain@....com, linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/5] mm: rmap: support batched unmapping for file large folios

On Wed, Jan 7, 2026 at 2:22 AM Wei Yang <richard.weiyang@...il.com> wrote:
>
> On Fri, Dec 26, 2025 at 02:07:59PM +0800, Baolin Wang wrote:
> >Similar to folio_referenced_one(), we can apply batched unmapping for file
> >large folios to optimize the performance of file folios reclamation.
> >
> >Barry previously implemented batched unmapping for lazyfree anonymous large
> >folios[1] and did not further optimize anonymous large folios or file-backed
> >large folios at that stage. As for file-backed large folios, the batched
> >unmapping support is relatively straightforward, as we only need to clear
> >the consecutive (present) PTE entries for file-backed large folios.
> >
> >Performance testing:
> >Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to
> >reclaim 8G file-backed folios via the memory.reclaim interface. I can observe
> >75% performance improvement on my Arm64 32-core server (and 50%+ improvement
> >on my X86 machine) with this patch.
> >
> >W/o patch:
> >real    0m1.018s
> >user    0m0.000s
> >sys     0m1.018s
> >
> >W/ patch:
> >real   0m0.249s
> >user   0m0.000s
> >sys    0m0.249s
> >
> >[1] https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u
> >Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> >Acked-by: Barry Song <baohua@...nel.org>
> >Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> >---
> > mm/rmap.c | 7 ++++---
> > 1 file changed, 4 insertions(+), 3 deletions(-)
> >
> >diff --git a/mm/rmap.c b/mm/rmap.c
> >index 985ab0b085ba..e1d16003c514 100644
> >--- a/mm/rmap.c
> >+++ b/mm/rmap.c
> >@@ -1863,9 +1863,10 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
> >       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))
> >+      /* We only support lazyfree or file folios batching for now ... */
> >+      if (folio_test_anon(folio) && folio_test_swapbacked(folio))
> >               return 1;
> >+
> >       if (pte_unused(pte))
> >               return 1;
> >
> >@@ -2231,7 +2232,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >                        *
> >                        * See Documentation/mm/mmu_notifier.rst
> >                        */
> >-                      dec_mm_counter(mm, mm_counter_file(folio));
> >+                      add_mm_counter(mm, mm_counter_file(folio), -nr_pages);
> >               }
> > discard:
> >               if (unlikely(folio_test_hugetlb(folio))) {
> >--
> >2.47.3
> >
>
> Hi, Baolin
>
> When reading your patch, I come up one small question.
>
> Current try_to_unmap_one() has following structure:
>
>     try_to_unmap_one()
>         while (page_vma_mapped_walk(&pvmw)) {
>             nr_pages = folio_unmap_pte_batch()
>
>             if (nr_pages = folio_nr_pages(folio))
>                 goto walk_done;
>         }
>
> I am thinking what if nr_pages > 1 but nr_pages != folio_nr_pages().
>
> If my understanding is correct, page_vma_mapped_walk() would start from
> (pvmw->address + PAGE_SIZE) in next iteration, but we have already cleared to
> (pvmw->address + nr_pages * PAGE_SIZE), right?
>
> Not sure my understanding is correct, if so do we have some reason not to
> skip the cleared range?

I don’t quite understand your question. For nr_pages > 1 but not equal
to nr_pages, page_vma_mapped_walk will skip the nr_pages - 1 PTEs inside.

take a look:

next_pte:
                do {
                        pvmw->address += PAGE_SIZE;
                        if (pvmw->address >= end)
                                return not_found(pvmw);
                        /* Did we cross page table boundary? */
                        if ((pvmw->address & (PMD_SIZE - PAGE_SIZE)) == 0) {
                                if (pvmw->ptl) {
                                        spin_unlock(pvmw->ptl);
                                        pvmw->ptl = NULL;
                                }
                                pte_unmap(pvmw->pte);
                                pvmw->pte = NULL;
                                pvmw->flags |= PVMW_PGTABLE_CROSSED;
                                goto restart;
                        }
                        pvmw->pte++;
                } while (pte_none(ptep_get(pvmw->pte)));


>
> --
> Wei Yang
> Help you, Help me

Thanks
Barry