block: use SLAB_TYPESAFE_BY_RCU for the bio slab

This flags ensures that the pages will not be reused for non-bio
allocations before the end of an RCU grace period.  With that we can
safely use a RCU lookup for bio polling as long as we are fine with
occasionally polling the wrong device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Mark Wunderlich <mark.wunderlich@intel.com>
Link: https://lore.kernel.org/r/20211012111226.760968-13-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -87,7 +87,8 @@ static struct bio_slab *create_bio_slab(unsigned int size)
 
 	snprintf(bslab->name, sizeof(bslab->name), "bio-%d", size);
 	bslab->slab = kmem_cache_create(bslab->name, size,
-			ARCH_KMALLOC_MINALIGN, SLAB_HWCACHE_ALIGN, NULL);
+			ARCH_KMALLOC_MINALIGN,
+			SLAB_HWCACHE_ALIGN | SLAB_TYPESAFE_BY_RCU, NULL);
 	if (!bslab->slab)
 		goto fail_alloc_slab;