linux-kernel - Re: [PATCH v6 2/2] mm/zswap: remove the memcpy if acomp is not sleepable

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Date: Sat, 24 Feb 2024 08:53:46 -0800
From: Chris Li <chrisl@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, davem@...emloft.net, hannes@...xchg.org, 
	herbert@...dor.apana.org.au, linux-crypto@...r.kernel.org, linux-mm@...ck.org, 
	nphamcs@...il.com, yosryahmed@...gle.com, zhouchengming@...edance.com, 
	ddstreet@...e.org, linux-kernel@...r.kernel.org, sjenning@...hat.com, 
	vitaly.wool@...sulko.com, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v6 2/2] mm/zswap: remove the memcpy if acomp is not sleepable

Acked-by: Chris Li <chrisl@...nel.org>

Chris

On Thu, Feb 22, 2024 at 12:12 AM Barry Song <21cnbao@...il.com> wrote:
>
> From: Barry Song <v-songbaohua@...o.com>
>
> Most compressors are actually CPU-based and won't sleep during
> compression and decompression. We should remove the redundant
> memcpy for them.
> This patch checks if the algorithm is sleepable by testing the
> CRYPTO_ALG_ASYNC algorithm flag.
> Generally speaking, async and sleepable are semantically similar
> but not equal. But for compress drivers, they are basically equal
> at least due to the below facts.
> Firstly, scompress drivers - crypto/deflate.c, lz4.c, zstd.c,
> lzo.c etc have no sleep. Secondly, zRAM has been using these
> scompress drivers for years in atomic contexts, and never
> worried those drivers going to sleep.
> One exception is that an async driver can sometimes still return
> synchronously per Herbert's clarification. In this case, we are
> still having a redundant memcpy. But we can't know if one
> particular acomp request will sleep or not unless crypto can
> expose more details for each specific request from offload
> drivers.
>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> Tested-by: Chengming Zhou <zhouchengming@...edance.com>
> Reviewed-by: Nhat Pham <nphamcs@...il.com>
> Acked-by: Yosry Ahmed <yosryahmed@...gle.com>
> Reviewed-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  mm/zswap.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 011e068eb355..de3c9e30bed7 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -162,6 +162,7 @@ struct crypto_acomp_ctx {
>         struct crypto_wait wait;
>         u8 *buffer;
>         struct mutex mutex;
> +       bool is_sleepable;
>  };
>
>  /*
> @@ -950,6 +951,7 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
>                 goto acomp_fail;
>         }
>         acomp_ctx->acomp = acomp;
> +       acomp_ctx->is_sleepable = acomp_is_async(acomp);
>
>         req = acomp_request_alloc(acomp_ctx->acomp);
>         if (!req) {
> @@ -1077,7 +1079,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
>         mutex_lock(&acomp_ctx->mutex);
>
>         src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> -       if (!zpool_can_sleep_mapped(zpool)) {
> +       if (acomp_ctx->is_sleepable && !zpool_can_sleep_mapped(zpool)) {
>                 memcpy(acomp_ctx->buffer, src, entry->length);
>                 src = acomp_ctx->buffer;
>                 zpool_unmap_handle(zpool, entry->handle);
> @@ -1091,7 +1093,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
>         BUG_ON(acomp_ctx->req->dlen != PAGE_SIZE);
>         mutex_unlock(&acomp_ctx->mutex);
>
> -       if (zpool_can_sleep_mapped(zpool))
> +       if (!acomp_ctx->is_sleepable || zpool_can_sleep_mapped(zpool))
>                 zpool_unmap_handle(zpool, entry->handle);
>  }
>
> --
> 2.34.1
>