linux-kernel - Re: [PATCH v13 3/9] block: add emulation for copy

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Message-ID: <20230801130702.2taecrgn4v66ehtx@green245>
Date:   Tue, 1 Aug 2023 18:37:02 +0530
From:   Nitesh Shetty <nj.shetty@...sung.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
        Keith Busch <kbusch@...nel.org>,
        Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        willy@...radead.org, hare@...e.de, djwong@...nel.org,
        bvanassche@....org, ming.lei@...hat.com, dlemoal@...nel.org,
        nitheshshetty@...il.com, gost.dev@...sung.com,
        Vincent Fu <vincent.fu@...sung.com>,
        Anuj Gupta <anuj20.g@...sung.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v13 3/9] block: add emulation for copy

On 23/07/20 09:50AM, Christoph Hellwig wrote:
>> +static void *blkdev_copy_alloc_buf(sector_t req_size, sector_t *alloc_size,
>> +		gfp_t gfp_mask)
>> +{
>> +	int min_size = PAGE_SIZE;
>> +	void *buf;
>> +
>> +	while (req_size >= min_size) {
>> +		buf = kvmalloc(req_size, gfp_mask);
>> +		if (buf) {
>> +			*alloc_size = req_size;
>> +			return buf;
>> +		}
>> +		/* retry half the requested size */
>> +		req_size >>= 1;
>> +	}
>> +
>> +	return NULL;
>
>Is there any good reason for using vmalloc instead of a bunch
>of distcontiguous pages?
>

kvmalloc seemed convenient for the purpose. 
We will need to call alloc_page in a loop to guarantee discontigous pages. 
Do you prefer that over kvmalloc?

>> +		ctx = kzalloc(sizeof(struct copy_ctx), gfp_mask);
>> +		if (!ctx)
>> +			goto err_ctx;
>
>I'd suspect it would be better to just allocte a single buffer and
>only have a single outstanding copy.  That will reduce the bandwith
>you can theoretically get, but copies tend to be background operations
>anyway.  It will reduce the required memory, and thus the chance for
>this operation to fail on a loaded system.  It will also dramatically
>reduce the effect on memory managment.
>

Next version will have that change.

Thank You,
Nitesh Shetty