Re: [PATCH v2 1/6] mm/zswap: change dstmem size to one page

[Chris Li]

Acked-by: Chris Li <chrisl@xxxxxxxxxx> (Google)

Chris

On Mon, Dec 18, 2023 at 12:22 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> Change the dstmem size from 2 * PAGE_SIZE to only one page since
> we only need at most one page when compress, and the "dlen" is also
> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
> we don't wanna store the output in zswap anyway.
>
> So change it to one page, and delete the stale comment.
>
> There is no any history about the reason why we needed 2 pages, it has
> been 2 * PAGE_SIZE since the time zswap was first merged.
>
> According to Yosry and Nhat, one potential reason is that we used to
> store a zswap header containing the swap entry in the compressed page
> for writeback purposes, but we don't do that anymore.
>
> This patch works good in kernel build testing even when the input data
> doesn't compress at all (i.e. dlen == PAGE_SIZE), which we can see
> from the bpftrace tool:
>
> bpftrace -e 'k:zpool_malloc {@[(uint32)arg1==4096]=count()}'
> @[1]: 2
> @[0]: 12011430
>
> Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
>  mm/zswap.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 7ee54a3d8281..976f278aa507 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -707,7 +707,7 @@ static int zswap_dstmem_prepare(unsigned int cpu)
>         struct mutex *mutex;
>         u8 *dst;
>
> -       dst = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
> +       dst = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
>         if (!dst)
>                 return -ENOMEM;
>
> @@ -1662,8 +1662,7 @@ bool zswap_store(struct folio *folio)
>         sg_init_table(&input, 1);
>         sg_set_page(&input, page, PAGE_SIZE, 0);
>
> -       /* zswap_dstmem is of size (PAGE_SIZE * 2). Reflect same in sg_list */
> -       sg_init_one(&output, dst, PAGE_SIZE * 2);
> +       sg_init_one(&output, dst, PAGE_SIZE);
>         acomp_request_set_params(acomp_ctx->req, &input, &output, PAGE_SIZE, dlen);
>         /*
>          * it maybe looks a little bit silly that we send an asynchronous request,
>
> --
> b4 0.10.1