[RFC PATCH 3/3] mm: entirely remove lru_add_drain in do_swap_page

[Barry Song (Xiaomi)]

We are doing a lot of redundant lru_add_drain() calls in
do_swap_page(), especially for synchronous I/O devices. For
example, the test program below currently ends up draining
lru_cache 100% of the time:

int main(int argc, char *argv[])
{
        int i;
 #define SIZE 100*1024*1024
	while(1) {
		volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

		for (int i = 0; i < SIZE/sizeof(int); i++)
			p[i] =  i%64;
		madvise((void *)p, SIZE, MADV_PAGEOUT);
		for (int i = 0; i < SIZE/sizeof(int); i++)
			p[i] =  i%64;
		munmap(p, SIZE);
	}
	return 0;
}

Folio reuse now relies primarily on the exclusive hint, making
lru_cache draining to drop the refcount in lru_cache largely
irrelevant.

Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
---
 mm/memory.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ce8ef27e7a54..b5a78670bcc8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4903,16 +4903,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	} else if (folio != swapcache)
 		page = folio_page(folio, 0);
 
-	/*
-	 * If we want to map a page that's in the swapcache writable, we
-	 * have to detect via the refcount if we're really the exclusive
-	 * owner. Try removing the extra reference from the local LRU
-	 * caches if required.
-	 */
-	if ((vmf->flags & FAULT_FLAG_WRITE) &&
-	    !folio_test_ksm(folio) && !folio_test_lru(folio))
-		lru_add_drain();
-
 	folio_throttle_swaprate(folio, GFP_KERNEL);
 
 	/*
-- 
2.39.3 (Apple Git-146)