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Date: Sat, 1 Jun 2024 08:18:39 +0200
From: Christoph Hellwig <hch@lst.de>
To: Nitesh Shetty <nj.shetty@samsung.com>
Cc: Jens Axboe <axboe@kernel.dk>, Jonathan Corbet <corbet@lwn.net>,
	Alasdair Kergon <agk@redhat.com>, Mike Snitzer <snitzer@kernel.org>,
	Mikulas Patocka <mpatocka@redhat.com>,
	Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Sagi Grimberg <sagi@grimberg.me>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
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	hare@suse.de, damien.lemoal@opensource.wdc.com,
	anuj20.g@samsung.com, joshi.k@samsung.com, nitheshshetty@gmail.com,
	gost.dev@samsung.com, Vincent Fu <vincent.fu@samsung.com>,
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Subject: Re: [PATCH v20 04/12] block: add emulation for copy
Message-ID: <20240601061839.GA6221@lst.de>
References: <20240520102033.9361-1-nj.shetty@samsung.com> <CGME20240520102906epcas5p15b5a0b3c8edd0bf3073030a792a328bb@epcas5p1.samsung.com> <20240520102033.9361-5-nj.shetty@samsung.com>
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On Mon, May 20, 2024 at 03:50:17PM +0530, Nitesh Shetty wrote:
> For the devices which does not support copy, copy emulation is added.
> It is required for in-kernel users like fabrics, where file descriptor is
> not available and hence they can't use copy_file_range.
> Copy-emulation is implemented by reading from source into memory and
> writing to the corresponding destination.
> At present in kernel user of emulation is fabrics.

I still don't see the point of offering this in the block layer,
at least in this form.  Caller usually can pre-allocate a buffer
if they need regular copies instead of doing constant allocation
and freeing which puts a lot of stress on the page allocator.

> +static void *blkdev_copy_alloc_buf(ssize_t req_size, ssize_t *alloc_size,
> +				   gfp_t gfp)
> +{
> +	int min_size = PAGE_SIZE;
> +	char *buf;
> +
> +	while (req_size >= min_size) {
> +		buf = kvmalloc(req_size, gfp);
> +		if (buf) {
> +			*alloc_size = req_size;
> +			return buf;
> +		}
> +		req_size >>= 1;
> +	}
> +
> +	return NULL;

And requiring a kernel mapping for data is is never used through the
kernel mapping is pretty silly as well.