On 31.05.24 12:48, 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>
---
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.
--
Cheers,
David / dhildenb
