netdev - Re: [PATCH v30 03/20] iov_iter: skip copy if src == dst for direct data placement

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Message-ID: <59fd61cc-4755-4619-bdb2-6b2091abf002@kernel.dk>
Date: Tue, 15 Jul 2025 09:06:41 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Aurelien Aptel <aaptel@...dia.com>, linux-nvme@...ts.infradead.org,
 netdev@...r.kernel.org, sagi@...mberg.me, hch@....de, kbusch@...nel.org,
 axboe@...com, chaitanyak@...dia.com, davem@...emloft.net, kuba@...nel.org
Cc: aurelien.aptel@...il.com, smalin@...dia.com, malin1024@...il.com,
 ogerlitz@...dia.com, yorayz@...dia.com, borisp@...dia.com,
 galshalom@...dia.com, mgurtovoy@...dia.com, tariqt@...dia.com,
 gus@...labora.com, viro@...iv.linux.org.uk, akpm@...ux-foundation.org
Subject: Re: [PATCH v30 03/20] iov_iter: skip copy if src == dst for direct
 data placement

On 7/15/25 7:27 AM, Aurelien Aptel wrote:
> From: Ben Ben-Ishay <benishay@...dia.com>
> 
> When using direct data placement (DDP) the NIC could write the payload
> directly into the destination buffer and constructs SKBs such that
> they point to this data. To skip copies when SKB data already resides
> in the destination buffer we check if (src == dst), and skip the copy
> when it's true.
> 
> Signed-off-by: Ben Ben-Ishay <benishay@...dia.com>
> Signed-off-by: Boris Pismenny <borisp@...dia.com>
> Signed-off-by: Or Gerlitz <ogerlitz@...dia.com>
> Signed-off-by: Yoray Zack <yorayz@...dia.com>
> Signed-off-by: Shai Malin <smalin@...dia.com>
> Signed-off-by: Aurelien Aptel <aaptel@...dia.com>
> Reviewed-by: Chaitanya Kulkarni <kch@...dia.com>
> Reviewed-by: Max Gurtovoy <mgurtovoy@...dia.com>
> ---
>  lib/iov_iter.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index f9193f952f49..47fdb32653a2 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -62,7 +62,14 @@ static __always_inline
>  size_t memcpy_to_iter(void *iter_to, size_t progress,
>  		      size_t len, void *from, void *priv2)
>  {
> -	memcpy(iter_to, from + progress, len);
> +	/*
> +	 * When using direct data placement (DDP) the hardware writes
> +	 * data directly to the destination buffer, and constructs
> +	 * IOVs such that they point to this data.
> +	 * Thus, when the src == dst we skip the memcpy.
> +	 */
> +	if (!(IS_ENABLED(CONFIG_ULP_DDP) && iter_to == from + progress))
> +		memcpy(iter_to, from + progress, len);
>  	return 0;
>  }

This seems like entirely the wrong place to apply this logic...

-- 
Jens Axboe