brd: support discard

Support discard requests in brd by zeroing or deleting the underlying backing
pages. This is simply to help with testing and documentation nature of
brd code.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 52 insertions, 1 deletions

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index 6081e81d5738..f1bf79d9bc0a 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -133,6 +133,28 @@ static struct page *brd_insert_page(struct brd_device *brd, sector_t sector)
         return page;
 }
 
+static void brd_free_page(struct brd_device *brd, sector_t sector)
+{
+        struct page *page;
+        pgoff_t idx;
+
+        spin_lock(&brd->brd_lock);
+        idx = sector >> PAGE_SECTORS_SHIFT;
+        page = radix_tree_delete(&brd->brd_pages, idx);
+        spin_unlock(&brd->brd_lock);
+        if (page)
+                __free_page(page);
+}
+
+static void brd_zero_page(struct brd_device *brd, sector_t sector)
+{
+        struct page *page;
+
+        page = brd_lookup_page(brd, sector);
+        if (page)
+                clear_highpage(page);
+}
+
 /*
  * Free all backing store pages and radix tree. This must only be called when
  * there are no other users of the device.
@@ -189,6 +211,24 @@ static int copy_to_brd_setup(struct brd_device *brd, sector_t sector, size_t n)
         return 0;
 }
 
+static void discard_from_brd(struct brd_device *brd,
+                        sector_t sector, size_t n)
+{
+        while (n >= PAGE_SIZE) {
+                /*
+                 * Don't want to actually discard pages here because
+                 * re-allocating the pages can result in writeback
+                 * deadlocks under heavy load.
+                 */
+                if (0)
+                        brd_free_page(brd, sector);
+                else
+                        brd_zero_page(brd, sector);
+                sector += PAGE_SIZE >> SECTOR_SHIFT;
+                n -= PAGE_SIZE;
+        }
+}
+
 /*
  * Copy n bytes from src to the brd starting at sector. Does not sleep.
  */
@@ -300,6 +340,12 @@ static int brd_make_request(struct request_queue *q, struct bio *bio)
                                                 get_capacity(bdev->bd_disk))
                 goto out;
 
+        if (unlikely(bio_rw_flagged(bio, BIO_RW_DISCARD))) {
+                err = 0;
+                discard_from_brd(brd, sector, bio->bi_size);
+                goto out;
+        }
+
         rw = bio_rw(bio);
         if (rw == READA)
                 rw = READ;
@@ -320,7 +366,7 @@ out:
 }
 
 #ifdef CONFIG_BLK_DEV_XIP
-static int brd_direct_access (struct block_device *bdev, sector_t sector,
+static int brd_direct_access(struct block_device *bdev, sector_t sector,
                         void **kaddr, unsigned long *pfn)
 {
         struct brd_device *brd = bdev->bd_disk->private_data;
@@ -437,6 +483,11 @@ static struct brd_device *brd_alloc(int i)
         blk_queue_max_hw_sectors(brd->brd_queue, 1024);
         blk_queue_bounce_limit(brd->brd_queue, BLK_BOUNCE_ANY);
 
+        brd->brd_queue->limits.discard_granularity = PAGE_SIZE;
+        brd->brd_queue->limits.max_discard_sectors = UINT_MAX;
+        brd->brd_queue->limits.discard_zeroes_data = 1;
+        queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, brd->brd_queue);
+
         disk = brd->brd_disk = alloc_disk(1 << part_shift);
         if (!disk)
                 goto out_free_queue;