Re: [PATCH v2] mm: swap: reuse exclusive folio directly instead of wp page faults

From: David Hildenbrand
Date: Mon Jun 03 2024 - 05:02:15 EST


On 02.06.24 02:45, Barry Song wrote:
From: Barry Song <v-songbaohua@xxxxxxxx>

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@xxxxxxxx>
---
-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@xxxxxxxxx/

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@xxxxxxxxxx>

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