Re: [PATCH v4 1/6] mm/zswap: change dstmem size to one page

[Barry Song <21cnbao@gmail.com>]

On Wed, Dec 27, 2023 at 4:55 AM Chengming Zhou
<zhouchengming@bytedance.com> 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.

i remember there was an over-compression case,  that means the compressed
data can be bigger than the source data. the similar thing is also done in zram
drivers/block/zram/zcomp.c

int zcomp_compress(struct zcomp_strm *zstrm,
                const void *src, unsigned int *dst_len)
{
        /*
         * Our dst memory (zstrm->buffer) is always `2 * PAGE_SIZE' sized
         * because sometimes we can endup having a bigger compressed data
         * due to various reasons: for example compression algorithms tend
         * to add some padding to the compressed buffer. Speaking of padding,
         * comp algorithm `842' pads the compressed length to multiple of 8
         * and returns -ENOSP when the dst memory is not big enough, which
         * is not something that ZRAM wants to see. We can handle the
         * `compressed_size > PAGE_SIZE' case easily in ZRAM, but when we
         * receive -ERRNO from the compressing backend we can't help it
         * anymore. To make `842' happy we need to tell the exact size of
         * the dst buffer, zram_drv will take care of the fact that
         * compressed buffer is too big.
         */
        *dst_len = PAGE_SIZE * 2;

        return crypto_comp_compress(zstrm->tfm,
                        src, PAGE_SIZE,
                        zstrm->buffer, dst_len);
}


>
> 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@google.com>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> Acked-by: Chris Li <chrisl@kernel.org> (Google)
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
>  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
>

Thanks
Barry