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 <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20240429102308.147627-1-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index b900fe9e003080163b5247bf1888bc544c96765f..558d8e67056608090c6dd436c0d626c6c8ba8318 100644 (file)
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -222,6 +222,23 @@ out:
        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;
@@ -229,6 +246,12 @@ 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;
@@ -309,6 +332,9 @@ 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)