brd: implement discard support

The ramdisk memory utilization can only go up when data is written to
new pages. Implement discard to provide the possibility to reduce memory
usage for pages no longer in use. Aligned discards will free the
associated pages, if any, and determinisitically return zeroed data
until written again.

Signed-off-by: Keith Busch <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/brd.c

···
 	return err;
 }
 
+static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
+{
+	sector_t aligned_sector = (sector + PAGE_SECTORS) & ~PAGE_SECTORS;
+	struct page *page;
+
+	size -= (aligned_sector - sector) * SECTOR_SIZE;
+	xa_lock(&brd->brd_pages);
+	while (size >= PAGE_SIZE && aligned_sector < rd_size * 2) {
+		page = __xa_erase(&brd->brd_pages, aligned_sector >> PAGE_SECTORS_SHIFT);
+		if (page)
+			__free_page(page);
+		aligned_sector += PAGE_SECTORS;
+		size -= PAGE_SIZE;
+	}
+	xa_unlock(&brd->brd_pages);
+}
+
 static void brd_submit_bio(struct bio *bio)
 {
 	struct brd_device *brd = bio->bi_bdev->bd_disk->private_data;
 	sector_t sector = bio->bi_iter.bi_sector;
 	struct bio_vec bvec;
 	struct bvec_iter iter;
+
+	if (unlikely(op_is_discard(bio->bi_opf))) {
+		brd_do_discard(brd, sector, bio->bi_iter.bi_size);
+		bio_endio(bio);
+		return;
+	}
 
 	bio_for_each_segment(bvec, bio, iter) {
 		unsigned int len = bvec.bv_len;
···
 		 *  is harmless)
 		 */
 		.physical_block_size	= PAGE_SIZE,
+		.max_hw_discard_sectors	= UINT_MAX,
+		.max_discard_segments	= 1,
+		.discard_granularity	= PAGE_SIZE,
 	};
 
 	list_for_each_entry(brd, &brd_devices, brd_list)