[PATCH] brd: implement discard support

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

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 <kbusch@kernel.org>
---
 drivers/block/brd.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index e322cef6596bf..e741b0c3a4f79 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -240,6 +240,23 @@ static int brd_do_bvec(struct brd_device *brd, struct page *page,
 	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;
@@ -247,6 +264,12 @@ static void brd_submit_bio(struct bio *bio)
 	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;
 		int err;
@@ -327,6 +350,9 @@ static int brd_alloc(int i)
 		 *  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)
-- 
2.43.0