[dm-devel] [RFC PATCH v3 1/2] block: add simple copy support

[Mikulas Patocka]

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