lightnvm: pblk: allocate line map bitmaps using a mempool

Line map bitmap allocations are fairly large and can fail. Allocation
failures are fatal to pblk, stopping the write pipeline. To avoid this,
allocate the bitmaps using a mempool instead.

Mempool allocations never fail if called from a process context,
and pblk *should* only allocate map bitmaps in process context,
but keep the failure handling for robustness sake.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/pblk.h

@@ -530,6 +530,10 @@ struct pblk_line_mgmt {
 	struct pblk_emeta *eline_meta[PBLK_DATA_LINES];
 	unsigned long meta_bitmap;
 
+	/* Cache and mempool for map/invalid bitmaps */
+	struct kmem_cache *bitmap_cache;
+	mempool_t *bitmap_pool;
+
 	/* Helpers for fast bitmap calculations */
 	unsigned long *bb_template;
 	unsigned long *bb_aux;