Re: [PATCH v2 3/5] mm/shmem: introduce copy_zero_to_iter() for large zeroing

[Chi Zhiling]

On 6/1/26 9:22 PM, Matthew Wilcox wrote:
> On Mon, Jun 01, 2026 at 01:57:02PM +0800, Chi Zhiling wrote:
> > Currently, holes larger than PAGE_SIZE cannot be handled because
> > ZERO_PAGE is limited to a single page. Add copy_zero_to_iter() as a
> > wrapper to support copying larger zero ranges to the iterator.
>
> I think Hugh put this optimisation in the wrong place, and you're
> perpetuating that ;-)
>
> So perhaps we can start by moving this optimisation to lib/iov_iter.c?
> And then you can redo your optimisation on top of that.

Sure, it's good to see this optimization moved into another place :)

In that case, I'll assume iov_iter_zero() is already well optimized and 
drop copy_page_to_iter() in v3.

By the way, I assume the benefit of this optimization depends on the CPU 
architecture, right? If so, I wonder whether lib/iov_iter.c is the most 
appropriate place for it.

> diff --git a/mm/shmem.c b/mm/shmem.c
> index 3b5dc21b323c..112cae9f9e4f 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3427,19 +3427,7 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   			else
>   				ret = copy_page_to_iter(page, offset, nr, to);
>   			folio_put(folio);
> -		} else if (user_backed_iter(to)) {
> -			/*
> -			 * Copy to user tends to be so well optimized, but
> -			 * clear_user() not so much, that it is noticeably
> -			 * faster to copy the zero page instead of clearing.
> -			 */
> -			ret = copy_page_to_iter(ZERO_PAGE(0), offset, nr, to);
>   		} else {
> -			/*
> -			 * But submitting the same page twice in a row to
> -			 * splice() - or others? - can result in confusion:
> -			 * so don't attempt that optimization on pipes etc.
> -			 */
>   			ret = iov_iter_zero(nr, to);
>   		}

I'll include this change to v3.

Thanks,