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

[Barry Song <21cnbao@xxxxxxxxx>]

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>
---
 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;
 		}
 		rmap_flags |= RMAP_EXCLUSIVE;
 	}
-- 
2.34.1