[PATCH 2/3] brd: enable discard

[Mikulas Patocka <mpatocka@xxxxxxxxxx>]

This patch implements discard in the brd driver. We use RCU to free the
page, so that if there are any concurrent readers or writes, they won't
touch the page after it is freed.

Note that we replace "BUG_ON(!page);" with "if (page) ..." in copy_to_brd
- the page can't be NULL under normal circumstances, it can only be NULL
if REQ_OP_WRITE races with REQ_OP_DISCARD. If these two bios race with
each other on the same page, the result is undefined, so we can handle
this race condition just by skipping the copying.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

---
 drivers/block/brd.c |   85 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 74 insertions(+), 11 deletions(-)

Index: linux-2.6/drivers/block/brd.c
===================================================================
--- linux-2.6.orig/drivers/block/brd.c
+++ linux-2.6/drivers/block/brd.c
@@ -46,6 +46,8 @@ struct brd_device {
 	u64			brd_nr_pages;
 };
 
+static bool discard;
+
 /*
  * Look up and return a brd's page for a given sector.
  */
@@ -100,6 +102,26 @@ static int brd_insert_page(struct brd_de
 	return ret;
 }
 
+static void brd_free_page_rcu(struct rcu_head *head)
+{
+	struct page *page = container_of(head, struct page, rcu_head);
+	__free_page(page);
+}
+
+static void brd_free_page(struct brd_device *brd, sector_t sector)
+{
+	struct page *page;
+	pgoff_t idx;
+
+	idx = sector >> PAGE_SECTORS_SHIFT;
+	page = xa_erase(&brd->brd_pages, idx);
+
+	if (page) {
+		BUG_ON(page->index != idx);
+		call_rcu(&page->rcu_head, brd_free_page_rcu);
+	}
+}
+
 /*
  * Free all backing store pages and xarray. This must only be called when
  * there are no other users of the device.
@@ -152,11 +174,11 @@ static void copy_to_brd(struct brd_devic
 	copy = min_t(size_t, n, PAGE_SIZE - offset);
 	rcu_read_lock();
 	page = brd_lookup_page(brd, sector);
-	BUG_ON(!page);
-
-	dst = kmap_atomic(page);
-	memcpy(dst + offset, src, copy);
-	kunmap_atomic(dst);
+	if (page) {
+		dst = kmap_atomic(page);
+		memcpy(dst + offset, src, copy);
+		kunmap_atomic(dst);
+	}
 	rcu_read_unlock();
 
 	if (copy < n) {
@@ -165,11 +187,11 @@ static void copy_to_brd(struct brd_devic
 		copy = n - copy;
 		rcu_read_lock();
 		page = brd_lookup_page(brd, sector);
-		BUG_ON(!page);
-
-		dst = kmap_atomic(page);
-		memcpy(dst, src, copy);
-		kunmap_atomic(dst);
+		if (page) {
+			dst = kmap_atomic(page);
+			memcpy(dst, src, copy);
+			kunmap_atomic(dst);
+		}
 		rcu_read_unlock();
 	}
 }
@@ -248,13 +270,44 @@ out:
 	return err;
 }
 
+void brd_do_discard(struct brd_device *brd, struct bio *bio)
+{
+	sector_t sector, len, front_pad;
+
+	if (unlikely(!discard)) {
+		bio->bi_status = BLK_STS_NOTSUPP;
+		return;
+	}
+
+	sector = bio->bi_iter.bi_sector;
+	len = bio_sectors(bio);
+	front_pad = -sector & (PAGE_SECTORS - 1);
+	sector += front_pad;
+	if (unlikely(len <= front_pad))
+		return;
+	len -= front_pad;
+	len = round_down(len, PAGE_SECTORS);
+	while (len) {
+		brd_free_page(brd, sector);
+		sector += PAGE_SECTORS;
+		len -= PAGE_SECTORS;
+		cond_resched();
+	}
+}
+
 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;
+	sector_t sector;
 	struct bio_vec bvec;
 	struct bvec_iter iter;
 
+	if (bio_op(bio) == REQ_OP_DISCARD) {
+		brd_do_discard(brd, bio);
+		goto endio;
+	}
+
+	sector = bio->bi_iter.bi_sector;
 	bio_for_each_segment(bvec, bio, iter) {
 		unsigned int len = bvec.bv_len;
 		int err;
@@ -276,6 +329,7 @@ static void brd_submit_bio(struct bio *b
 		sector += len >> SECTOR_SHIFT;
 	}
 
+endio:
 	bio_endio(bio);
 }
 
@@ -299,6 +353,10 @@ static int max_part = 1;
 module_param(max_part, int, 0444);
 MODULE_PARM_DESC(max_part, "Num Minors to reserve between devices");
 
+static bool discard = false;
+module_param(discard, bool, 0444);
+MODULE_PARM_DESC(discard, "Support discard");
+
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(RAMDISK_MAJOR);
 MODULE_ALIAS("rd");
@@ -364,6 +422,11 @@ static int brd_alloc(int i)
 	 */
 	blk_queue_physical_block_size(disk->queue, PAGE_SIZE);
 
+	if (discard) {
+		disk->queue->limits.discard_granularity = PAGE_SIZE;
+		blk_queue_max_discard_sectors(disk->queue, round_down(UINT_MAX, PAGE_SECTORS));
+	}
+
 	/* Tell the block layer that this is not a rotational device */
 	blk_queue_flag_set(QUEUE_FLAG_NONROT, disk->queue);
 	blk_queue_flag_set(QUEUE_FLAG_SYNCHRONOUS, disk->queue);