netdev - Re: [PATCH v5 net-next 02/36] iov_iter: DDP copy to iter/pages

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Date:   Fri, 23 Jul 2021 05:21:18 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Boris Pismenny <borispismenny@...il.com>,
        Boris Pismenny <borisp@...dia.com>, dsahern@...il.com,
        kuba@...nel.org, davem@...emloft.net, saeedm@...dia.com,
        sagi@...mberg.me, axboe@...com, kbusch@...nel.org,
        edumazet@...gle.com, smalin@...vell.com, boris.pismenny@...il.com,
        linux-nvme@...ts.infradead.org, netdev@...r.kernel.org,
        benishay@...dia.com, ogerlitz@...dia.com, yorayz@...dia.com,
        Boris Pismenny <borisp@...lanox.com>,
        Ben Ben-Ishay <benishay@...lanox.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Yoray Zack <yorayz@...lanox.com>
Subject: Re: [PATCH v5 net-next 02/36] iov_iter: DDP copy to iter/pages

On Fri, Jul 23, 2021 at 07:03:02AM +0200, Christoph Hellwig wrote:
> On Thu, Jul 22, 2021 at 11:23:38PM +0300, Boris Pismenny wrote:
> > This routine, like other changes in this file, replicates the logic in
> > memcpy_to_page. The only difference is that "ddp" avoids copies when the
> > copy source and destinations buffers are one and the same.
> 
> Now why can't we just make that change to the generic routine?

Doable... replace memcpy(base, addr + off, len) with
	base != addr + off && memcpy(base, addr + off, len)
in _copy_to_iter() and be done with that...