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Date:   Thu, 22 Jul 2021 20:55:30 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Boris Pismenny <borisp@nvidia.com>
Cc:     dsahern@gmail.com, kuba@kernel.org, davem@davemloft.net,
        saeedm@nvidia.com, hch@lst.de, sagi@grimberg.me, axboe@fb.com,
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        yorayz@nvidia.com, Boris Pismenny <borisp@mellanox.com>,
        Ben Ben-Ishay <benishay@mellanox.com>,
        Or Gerlitz <ogerlitz@mellanox.com>,
        Yoray Zack <yorayz@mellanox.com>
Subject: Re: [PATCH v5 net-next 02/36] iov_iter: DDP copy to iter/pages
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References: <20210722110325.371-1-borisp@nvidia.com>
 <20210722110325.371-3-borisp@nvidia.com>
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On Thu, Jul 22, 2021 at 02:02:51PM +0300, Boris Pismenny wrote:
> From: Boris Pismenny <borisp@mellanox.com>
> 
> When using direct data placement (DDP) the NIC writes some of the payload
> directly to the destination buffer, and constructs SKBs such that they
> point to this data. To skip copies when SKB data already resides in the
> destination we use the newly introduced routines in this commit, which
> check if (src == dst), and skip the copy when that's true.
> 
> As the current user for these routines is in the block layer (nvme-tcp),
> then we only apply the change for bio_vec. Other routines use the normal
> methods for copying.

Please, take a look at -rc1 and see the changes in lib/iov_iter.c in there.