[dm-devel] [PATCH 3/4] brd: enable discard

[Mikulas Patocka]

On Tue, 20 Sep 2022, Christoph Hellwig wrote:

> > @@ -289,6 +308,23 @@ static void brd_submit_bio(struct bio *b
> >  	struct bio_vec bvec;
> >  	struct bvec_iter iter;
> >  
> > +	if (bio_op(bio) == REQ_OP_DISCARD) {
> > +		sector_t len = bio_sectors(bio);
> > +		sector_t front_pad = -sector & (PAGE_SECTORS - 1);
> > +		sector += front_pad;
> > +		if (unlikely(len <= front_pad))
> > +			goto endio;
> > +		len -= front_pad;
> > +		len = round_down(len, PAGE_SECTORS);
> > +		while (len) {
> > +			brd_free_page(brd, sector);
> > +			sector += PAGE_SECTORS;
> > +			len -= PAGE_SECTORS;
> > +			cond_resched();
> > +		}
> > +		goto endio;
> > +	}
> > +
> >  	bio_for_each_segment(bvec, bio, iter) {
> 
> Please add separate helpers to each type of IO and just make the
> main submit_bio method a dispatch on the types instead of this
> spaghetti code.
> 
> > +	disk->queue->limits.discard_granularity = PAGE_SIZE;
> > +	blk_queue_max_discard_sectors(disk->queue, UINT_MAX);
> 
> We'll probably want an opt in for this new feature.

OK. I addressed these concerns and I'll send a second version of the patch 
set.

Mikulas