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References: <20231213-zswap-dstmem-v1-0-896763369d04@bytedance.com> <20231213-zswap-dstmem-v1-1-896763369d04@bytedance.com>
In-Reply-To: <20231213-zswap-dstmem-v1-1-896763369d04@bytedance.com>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Wed, 13 Dec 2023 15:24:17 -0800
Message-ID: <CAJD7tkbicqevYuJbTUZBQUo2n_k-HKTixG5TbXGtdbtFJgFmfg@mail.gmail.com>
Subject: Re: [PATCH 1/5] mm/zswap: reuse dstmem when decompress
To:     Chengming Zhou <zhouchengming@bytedance.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Nhat Pham <nphamcs@gmail.com>, Chris Li <chriscli@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Tue, Dec 12, 2023 at 8:18=E2=80=AFPM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> In the !zpool_can_sleep_mapped() case such as zsmalloc, we need to first
> copy the entry->handle memory to a temporary memory, which is allocated
> using kmalloc.
>
> Obviously we can reuse the per-compressor dstmem to avoid allocating
> every time, since it's percpu-compressor and protected in mutex.
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> ---
>  mm/zswap.c | 29 +++++++++--------------------
>  1 file changed, 9 insertions(+), 20 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 7ee54a3d8281..edb8b45ed5a1 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1772,9 +1772,9 @@ bool zswap_load(struct folio *folio)
>         struct zswap_entry *entry;
>         struct scatterlist input, output;
>         struct crypto_acomp_ctx *acomp_ctx;
> -       u8 *src, *dst, *tmp;
> +       unsigned int dlen =3D PAGE_SIZE;
> +       u8 *src, *dst;
>         struct zpool *zpool;
> -       unsigned int dlen;
>         bool ret;
>
>         VM_WARN_ON_ONCE(!folio_test_locked(folio));
> @@ -1796,27 +1796,18 @@ bool zswap_load(struct folio *folio)
>                 goto stats;
>         }
>
> -       zpool =3D zswap_find_zpool(entry);
> -       if (!zpool_can_sleep_mapped(zpool)) {
> -               tmp =3D kmalloc(entry->length, GFP_KERNEL);
> -               if (!tmp) {
> -                       ret =3D false;
> -                       goto freeentry;
> -               }
> -       }
> -
>         /* decompress */
> -       dlen =3D PAGE_SIZE;
> -       src =3D zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> +       acomp_ctx =3D raw_cpu_ptr(entry->pool->acomp_ctx);
> +       mutex_lock(acomp_ctx->mutex);
>
> +       zpool =3D zswap_find_zpool(entry);
> +       src =3D zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
>         if (!zpool_can_sleep_mapped(zpool)) {
> -               memcpy(tmp, src, entry->length);
> -               src =3D tmp;
> +               memcpy(acomp_ctx->dstmem, src, entry->length);
> +               src =3D acomp_ctx->dstmem;

I don't like that we are now using acomp_ctx->dstmem and
acomp_ctx->mutex now for purposes other than what the naming suggests.

How about removing these two fields from acomp_ctx, and directly using
zswap_dstmem and zswap_mutex in both the load and store paths, rename
them, and add proper comments above their definitions that they are
for generic percpu buffering on the load and store paths?