linux-kernel - Re: [PATCH v2 2/3] mm/zswap: remove the memcpy if acomp is not sleepable

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Date: Fri, 16 Feb 2024 08:30:10 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
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, zhouchengming@...edance.com, chriscli@...gle.com, 
	chrisl@...nel.org, ddstreet@...e.org, linux-kernel@...r.kernel.org, 
	sjenning@...hat.com, vitaly.wool@...sulko.com, 
	Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v2 2/3] mm/zswap: remove the memcpy if acomp is not sleepable

On Fri, Feb 16, 2024 at 05:08:14PM +1300, Barry Song 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.
> 
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> Tested-by: Chengming Zhou <zhouchengming@...edance.com>
> Reviewed-by: Nhat Pham <nphamcs@...il.com>
> ---
>  mm/zswap.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 350dd2fc8159..6319d2281020 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -168,6 +168,7 @@ struct crypto_acomp_ctx {
>  	struct crypto_wait wait;
>  	u8 *buffer;
>  	struct mutex mutex;
> +	bool is_sleepable;
>  };
>  
>  /*
> @@ -716,6 +717,7 @@ 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_sleepable(acomp);

Just one question here. In patch 1, sleepable seems to mean "not async".
IIUC, even a synchronous algorithm may sleep (e.g. if there is a
cond_resched or waiting for a mutex). Does sleepable in acomp terms the
same as "atomic" in scheduling/preemption terms?

Also, was this tested with debug options to catch any possible sleeps in
atomic context?

If the answer to both questions is yes, the change otherwise LGTM. Feel
free to add:
Acked-by: Yosry Ahmed <yosryahmed@...gle.com>

Thanks!

>  
>  	req = acomp_request_alloc(acomp_ctx->acomp);
>  	if (!req) {
> @@ -1368,7 +1370,7 @@ static void __zswap_load(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);
> @@ -1382,7 +1384,7 @@ static void __zswap_load(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
>