From c18160dba5ff633f7ccd779f23ee97603eda0094 Mon Sep 17 00:00:00 2001 From: Barry Song Date: Sun, 2 Jun 2024 12:45:02 +1200 Subject: [PATCH] mm: swap: reuse exclusive folio directly instead of wp page faults After swapping out, we perform a swap-in operation. If we first read and then write, we encounter a major fault in do_swap_page for reading, along with additional minor faults in do_wp_page for writing. However, the latter appears to be unnecessary and inefficient. Instead, we can directly reuse in do_swap_page and completely eliminate the need for do_wp_page. This patch achieves that optimization specifically for exclusive folios. The following microbenchmark demonstrates the significant reduction in minor faults. #define DATA_SIZE (2UL * 1024 * 1024) #define PAGE_SIZE (4UL * 1024) static void *read_write_data(char *addr) { char tmp; for (int i = 0; i < DATA_SIZE; i += PAGE_SIZE) { tmp = *(volatile char *)(addr + i); *(volatile char *)(addr + i) = tmp; } } int main(int argc, char **argv) { struct rusage ru; char *addr = mmap(NULL, DATA_SIZE, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); memset(addr, 0x11, DATA_SIZE); do { long old_ru_minflt, old_ru_majflt; long new_ru_minflt, new_ru_majflt; madvise(addr, DATA_SIZE, MADV_PAGEOUT); getrusage(RUSAGE_SELF, &ru); old_ru_minflt = ru.ru_minflt; old_ru_majflt = ru.ru_majflt; read_write_data(addr); getrusage(RUSAGE_SELF, &ru); new_ru_minflt = ru.ru_minflt; new_ru_majflt = ru.ru_majflt; printf("minor faults:%ld major faults:%ld\n", new_ru_minflt - old_ru_minflt, new_ru_majflt - old_ru_majflt); } while(0); return 0; } w/o patch, / # ~/a.out minor faults:512 major faults:512 w/ patch, / # ~/a.out minor faults:0 major faults:512 Minor faults decrease to 0! Link: https://lkml.kernel.org/r/20240602004502.26895-1-21cnbao@gmail.com Signed-off-by: Barry Song Acked-by: David Hildenbrand Cc: Chris Li Cc: Kairui Song Cc: Matthew Wilcox (Oracle) Cc: Minchan Kim Cc: Ryan Roberts Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- mm/memory.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index a618fef8d47e..edd987f1bd8c 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4310,6 +4310,10 @@ check_folio: add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages); add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages); pte = mk_pte(page, vma->vm_page_prot); + if (pte_swp_soft_dirty(vmf->orig_pte)) + pte = pte_mksoft_dirty(pte); + if (pte_swp_uffd_wp(vmf->orig_pte)) + pte = pte_mkuffd_wp(pte); /* * Same logic as in do_wp_page(); however, optimize for pages that are @@ -4319,18 +4323,18 @@ check_folio: */ if (!folio_test_ksm(folio) && (exclusive || folio_ref_count(folio) == 1)) { - if (vmf->flags & FAULT_FLAG_WRITE) { - pte = maybe_mkwrite(pte_mkdirty(pte), vma); - vmf->flags &= ~FAULT_FLAG_WRITE; + if ((vma->vm_flags & VM_WRITE) && !userfaultfd_pte_wp(vma, pte) && + !vma_soft_dirty_enabled(vma)) { + pte = pte_mkwrite(pte, vma); + if (vmf->flags & FAULT_FLAG_WRITE) { + pte = pte_mkdirty(pte); + vmf->flags &= ~FAULT_FLAG_WRITE; + } } rmap_flags |= RMAP_EXCLUSIVE; } folio_ref_add(folio, nr_pages - 1); flush_icache_pages(vma, page, nr_pages); - if (pte_swp_soft_dirty(vmf->orig_pte)) - pte = pte_mksoft_dirty(pte); - if (pte_swp_uffd_wp(vmf->orig_pte)) - pte = pte_mkuffd_wp(pte); vmf->orig_pte = pte_advance_pfn(pte, page_idx); /* ksm created a completely new copy */