Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation
From: David Hildenbrand <hidden>
Date: 2025-06-25 11:01:36
Also in:
linux-mm, linux-riscv, lkml
Subsystem:
memory management, memory management - rmap (reverse mapping), the rest · Maintainers:
Andrew Morton, David Hildenbrand, Lorenzo Stoakes, Linus Torvalds
On 25.06.25 12:57, Barry Song wrote:
quoted
quoted
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?That's a good point about the "all-or-nothing" batching logic ;) It seems the "all-or-nothing" approach is specific to the lazyfree use case, which needs to unmap the entire folio for reclamation. If that's not possible, it falls back to the single-page slow path.Other cases advance the PTE themselves, while try_to_unmap_one() relies on page_vma_mapped_walk() to advance the PTE. Unless we want to manually modify pvmw.pte and pvmw.address outside of page_vma_mapped_walk(), which to me seems like a violation of layers. :-)
Please explain to me why the following is not clearer and better:
diff --git a/mm/rmap.c b/mm/rmap.c
index 8200d705fe4ac..09e2c2f28aa58 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c@@ -1845,23 +1845,31 @@ 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); + 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 folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL, - NULL, NULL) == max_nr; + return 1; + return folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags, + NULL, NULL, NULL); } /*
@@ -2024,9 +2032,7 @@ 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);
--
Cheers,
David / dhildenb