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

[Baoquan He <baoquan.he@linux.dev>]

On 06/11/26 at 06:51pm, Barry Song (Xiaomi) wrote:
> 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@kernel.org>
> ---
>  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);

Reviewed-by: Baoquan He <baoquan.he@linux.dev>

>  
>  	/*
> -- 
> 2.39.3 (Apple Git-146)
>