Commit 354dffd2 authored Feb 14, 2025 by Barry Song Committed by Andrew Morton Mar 16, 2025

mm: support batched unmap for lazyfree large folios during reclamation

Currently, the PTEs and rmap of a large folio are removed one at a time. 
This is not only slow but also causes the large folio to be unnecessarily
added to deferred_split, which can lead to races between the
deferred_split shrinker callback and memory reclamation.  This patch
releases all PTEs and rmap entries in a batch.  Currently, it only handles
lazyfree large folios.

The below microbench tries to reclaim 128MB lazyfree large folios
whose sizes are 64KiB:

 #include <stdio.h>
 #include <sys/mman.h>
 #include <string.h>
 #include <time.h>

 #define SIZE 128*1024*1024  // 128 MB

 unsigned long read_split_deferred()
 {
 	FILE *file = fopen("/sys/kernel/mm/transparent_hugepage"
			"/hugepages-64kB/stats/split_deferred", "r");
 	if (!file) {
 		perror("Error opening file");
 		return 0;
 	}

 	unsigned long value;
 	if (fscanf(file, "%lu", &value) != 1) {
 		perror("Error reading value");
 		fclose(file);
 		return 0;
 	}

 	fclose(file);
 	return value;
 }

 int main(int argc, char *argv[])
 {
 	while(1) {
 		volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
 				MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

 		memset((void *)p, 1, SIZE);

 		madvise((void *)p, SIZE, MADV_FREE);

 		clock_t start_time = clock();
 		unsigned long start_split = read_split_deferred();
 		madvise((void *)p, SIZE, MADV_PAGEOUT);
 		clock_t end_time = clock();
 		unsigned long end_split = read_split_deferred();

 		double elapsed_time = (double)(end_time - start_time) / CLOCKS_PER_SEC;
 		printf("Time taken by reclamation: %f seconds, split_deferred: %ld\n",
 			elapsed_time, end_split - start_split);

 		munmap((void *)p, SIZE);
 	}
 	return 0;
 }

w/o patch:
~ # ./a.out
Time taken by reclamation: 0.177418 seconds, split_deferred: 2048
Time taken by reclamation: 0.178348 seconds, split_deferred: 2048
Time taken by reclamation: 0.174525 seconds, split_deferred: 2048
Time taken by reclamation: 0.171620 seconds, split_deferred: 2048
Time taken by reclamation: 0.172241 seconds, split_deferred: 2048
Time taken by reclamation: 0.174003 seconds, split_deferred: 2048
Time taken by reclamation: 0.171058 seconds, split_deferred: 2048
Time taken by reclamation: 0.171993 seconds, split_deferred: 2048
Time taken by reclamation: 0.169829 seconds, split_deferred: 2048
Time taken by reclamation: 0.172895 seconds, split_deferred: 2048
Time taken by reclamation: 0.176063 seconds, split_deferred: 2048
Time taken by reclamation: 0.172568 seconds, split_deferred: 2048
Time taken by reclamation: 0.171185 seconds, split_deferred: 2048
Time taken by reclamation: 0.170632 seconds, split_deferred: 2048
Time taken by reclamation: 0.170208 seconds, split_deferred: 2048
Time taken by reclamation: 0.174192 seconds, split_deferred: 2048
...

w/ patch:
~ # ./a.out
Time taken by reclamation: 0.074231 seconds, split_deferred: 0
Time taken by reclamation: 0.071026 seconds, split_deferred: 0
Time taken by reclamation: 0.072029 seconds, split_deferred: 0
Time taken by reclamation: 0.071873 seconds, split_deferred: 0
Time taken by reclamation: 0.073573 seconds, split_deferred: 0
Time taken by reclamation: 0.071906 seconds, split_deferred: 0
Time taken by reclamation: 0.073604 seconds, split_deferred: 0
Time taken by reclamation: 0.075903 seconds, split_deferred: 0
Time taken by reclamation: 0.073191 seconds, split_deferred: 0
Time taken by reclamation: 0.071228 seconds, split_deferred: 0
Time taken by reclamation: 0.071391 seconds, split_deferred: 0
Time taken by reclamation: 0.071468 seconds, split_deferred: 0
Time taken by reclamation: 0.071896 seconds, split_deferred: 0
Time taken by reclamation: 0.072508 seconds, split_deferred: 0
Time taken by reclamation: 0.071884 seconds, split_deferred: 0
Time taken by reclamation: 0.072433 seconds, split_deferred: 0
Time taken by reclamation: 0.071939 seconds, split_deferred: 0
...

Link: https://lkml.kernel.org/r/20250214093015.51024-4-21cnbao@gmail.com


Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chis Li <chrisl@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gavin Shan <gshan@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mauricio Faria de Oliveira <mfo@canonical.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shaoqin Huang <shahuang@redhat.com>
Cc: Tangquan Zheng <zhengtangquan@oppo.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent 2f4ab3ac

mm/rmap.c

+50 −22

Original line number	Diff line number	Diff line
		@@ -1722,6 +1722,25 @@ void folio_remove_rmap_pmd(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)
		{
		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);

		if (!folio_test_anon(folio) \|\| folio_test_swapbacked(folio))
		return false;
		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;
		}

		/*
		* @arg: enum ttu_flags will be passed to this argument
		*/
		@@ -1735,6 +1754,7 @@ static bool try_to_unmap_one(struct folio folio, struct vm_area_struct vma,
		struct page *subpage;
		struct mmu_notifier_range range;
		enum ttu_flags flags = (enum ttu_flags)(long)arg;
		unsigned long nr_pages = 1, end_addr;
		unsigned long pfn;
		unsigned long hsz = 0;

		@@ -1874,9 +1894,14 @@ 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))) {
		flush_cache_page(vma, address, pfn);
		if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
		can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
		nr_pages = folio_nr_pages(folio);
		end_addr = address + nr_pages * PAGE_SIZE;
		flush_cache_range(vma, address, end_addr);

		/* Nuke the page table entry. */
		if (should_defer_flush(mm, flags)) {
		pteval = get_and_clear_full_ptes(mm, address, pvmw.pte, nr_pages, 0);
		/*
		* We clear the PTE but do not flush so potentially
		* a remote CPU could still be writing to the folio.
		@@ -1885,12 +1910,10 @@ static bool try_to_unmap_one(struct folio folio, struct vm_area_struct vma,
		* transition on a cached TLB entry is written through
		* and traps if the PTE is unmapped.
		*/
		pteval = ptep_get_and_clear(mm, address, pvmw.pte);

		set_tlb_ubc_flush_pending(mm, pteval, address, address + PAGE_SIZE);
		} else {
		pteval = ptep_clear_flush(vma, address, pvmw.pte);
		}
		if (should_defer_flush(mm, flags))
		set_tlb_ubc_flush_pending(mm, pteval, address, end_addr);
		else
		flush_tlb_range(vma, address, end_addr);
		if (pte_dirty(pteval))
		folio_mark_dirty(folio);
		} else {
		@@ -1968,7 +1991,7 @@ static bool try_to_unmap_one(struct folio folio, struct vm_area_struct vma,
		* redirtied either using the page table or a previously
		* obtained GUP reference.
		*/
		set_pte_at(mm, address, pvmw.pte, pteval);
		set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
		folio_set_swapbacked(folio);
		goto walk_abort;
		} else if (ref_count != 1 + map_count) {
		@@ -1981,10 +2004,10 @@ static bool try_to_unmap_one(struct folio folio, struct vm_area_struct vma,
		* We'll come back here later and detect if the folio was
		* dirtied when the additional reference is gone.
		*/
		set_pte_at(mm, address, pvmw.pte, pteval);
		set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
		goto walk_abort;
		}
		dec_mm_counter(mm, MM_ANONPAGES);
		add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
		goto discard;
		}

		@@ -2049,13 +2072,18 @@ static bool try_to_unmap_one(struct folio folio, struct vm_area_struct vma,
		dec_mm_counter(mm, mm_counter_file(folio));
		}
		discard:
		if (unlikely(folio_test_hugetlb(folio)))
		if (unlikely(folio_test_hugetlb(folio))) {
		hugetlb_remove_rmap(folio);
		else
		folio_remove_rmap_pte(folio, subpage, vma);
		} 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)
		goto walk_done;
		continue;
		walk_abort:
		ret = false;