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Date: Fri, 31 May 2024 23:55:51 +1200
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, chrisl@...nel.org,
surenb@...gle.com, kasong@...cent.com, minchan@...nel.org,
willy@...radead.org, ryan.roberts@....com, linux-kernel@...r.kernel.org,
Barry Song <v-songbaohua@...o.com>
Subject: Re: [RFC PATCH] mm: swap: reuse exclusive folio directly instead of
wp page faults
On Fri, May 31, 2024 at 11:08 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 31.05.24 12:48, 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>
> > ---
> > mm/memory.c | 7 ++++---
> > 1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index eef4e482c0c2..e1d2e339958e 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4325,9 +4325,10 @@ 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) {
> > + pte = pte_mkwrite(pte_mkdirty(pte), vma);
> > + if (vmf->flags & FAULT_FLAG_WRITE)
> > + vmf->flags &= ~FAULT_FLAG_WRITE;
>
> This implies, that even on a read fault, you would mark the pte dirty
> and it would have to be written back to swap if still in the swap cache
> and only read.
>
> That is controversial.
>
> What is less controversial is doing what mprotect() via
> change_pte_range()/can_change_pte_writable() would do: mark the PTE
> writable but not dirty.
>
> I suggest setting the pte only dirty if FAULT_FLAG_WRITE is set.
Thanks!
I assume you mean something as below?
diff --git a/mm/memory.c b/mm/memory.c
index eef4e482c0c2..dbf1ba8ccfd6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4317,6 +4317,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
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
* certainly not shared either because we just allocated them without
@@ -4325,18 +4329,19 @@ 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) {
+ if (vmf->flags & FAULT_FLAG_WRITE) {
+ pte = pte_mkwrite(pte_mkdirty(pte), vma);
+ vmf->flags &= ~FAULT_FLAG_WRITE;
+ } else if ((!vma_soft_dirty_enabled(vma) ||
pte_soft_dirty(pte))
+ && !userfaultfd_pte_wp(vma, pte)) {
+ pte = pte_mkwrite(pte, vma);
+ }
}
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 */
>
> --
> Cheers,
>
> David / dhildenb
>
Thanks
Barry
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