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Message-ID: <baf84b51-7e8a-4da8-9662-3f5cf14ad6f6@redhat.com>
Date: Mon, 3 Jun 2024 11:01:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
linux-mm@...ck.org
Cc: chrisl@...nel.org, kasong@...cent.com, linux-kernel@...r.kernel.org,
minchan@...nel.org, ryan.roberts@....com, surenb@...gle.com,
v-songbaohua@...o.com, willy@...radead.org
Subject: Re: [PATCH v2] mm: swap: reuse exclusive folio directly instead of wp
page faults
On 02.06.24 02:45, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
>
> 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!
>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
> -v2:
> * don't set the dirty flag for read fault, per David;
> * make write-protect of uffd_wp clear and remove confusion(
> it used to be "wrprotected->writable->wrprotected"), per
> David;
> Thank you for reviewing, David.
>
> -v1:
> https://lore.kernel.org/linux-mm/20240531104819.140218-1-21cnbao@gmail.com/
>
> mm/memory.c | 18 +++++++++++-------
> 1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index eef4e482c0c2..9696c7397b85 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4316,6 +4316,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> 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
> @@ -4325,18 +4329,18 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> */
> 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 */
Thanks, LGTM!
Acked-by: David Hildenbrand <david@...hat.com>
Maybe we really want (likely with better naming)
soft_dirty_pte(vma, pte)
soft_dirty_pte(vma, pmd)
that wrap the VMA+pte check, like we have for uffd-wp.
$ git grep vma_soft_dirty | grep -E "(pmd)|(pte)"
mm/gup.c: if (vma_soft_dirty_enabled(vma) && !pmd_soft_dirty(pmd))
mm/gup.c: if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte))
mm/huge_memory.c: if (vma_soft_dirty_enabled(vma) && !pmd_soft_dirty(pmd))
mm/mprotect.c: if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte))
... then we could simply use the one here as well.
--
Cheers,
David / dhildenb
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