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

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Message-ID: <20230802063124.4652m3gfbhdmghlt@moria.home.lan>
Date:   Wed, 2 Aug 2023 02:31:24 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Nitesh Shetty <nj.shetty@...sung.com>
Cc:     Christoph Hellwig <hch@....de>, 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 Tue, Aug 01, 2023 at 06:37:02PM +0530, Nitesh Shetty wrote:
> 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?

No, kvmalloc should be the preferred approach here now: with large
folios, we're now getting better about doing more large memory
allocations and avoiding fragmentation, so in practice this won't be a
vmalloc allocation except in exceptional circumstances, and performance
will be better and the code will be simpler doing a single large
allocation.