linux-kernel - Re: [RFC PATCH v3 1/2] block: add simple copy support

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Date:   Fri, 11 Dec 2020 12:03:52 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Johannes Thumshirn <Johannes.Thumshirn@....com>
cc:     SelvaKumar S <selvakuma.s1@...sung.com>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "kbusch@...nel.org" <kbusch@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        Damien Le Moal <Damien.LeMoal@....com>,
        "hch@....de" <hch@....de>, "sagi@...mberg.me" <sagi@...mberg.me>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
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        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
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        "joshi.k@...sung.com" <joshi.k@...sung.com>,
        "javier.gonz@...sung.com" <javier.gonz@...sung.com>
Subject: Re: [RFC PATCH v3 1/2] block: add simple copy support



On Fri, 11 Dec 2020, Johannes Thumshirn wrote:

> On 11/12/2020 15:57, SelvaKumar S wrote:
> [...] 
> > +int blk_copy_emulate(struct block_device *bdev, struct blk_copy_payload *payload,
> > +		gfp_t gfp_mask)
> > +{
> > +	struct request_queue *q = bdev_get_queue(bdev);
> > +	struct bio *bio;
> > +	void *buf = NULL;
> > +	int i, nr_srcs, max_range_len, ret, cur_dest, cur_size;
> > +
> > +	nr_srcs = payload->copy_range;
> > +	max_range_len = q->limits.max_copy_range_sectors << SECTOR_SHIFT;
> > +	cur_dest = payload->dest;
> > +	buf = kvmalloc(max_range_len, GFP_ATOMIC);
> 
> Why GFP_ATOMIC and not the passed in gfp_mask? Especially as this is a kvmalloc()
> which has the potential to grow quite big.

You are right, this is confusing.

There's this piece of code at the top of kvmalloc_node:
        if ((flags & GFP_KERNEL) != GFP_KERNEL)
                return kmalloc_node(size, flags, node);

So, when you use GFP_ATOMIC flag, it will always fall back to kmalloc.

Mikulas