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From:   Christoph Hellwig <hch@infradead.org>
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Zdenek Kabelac <zkabelac@redhat.com>,
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Subject: Re: [PATCH 3/4] brd: enable discard
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> @@ -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.