bcachefs: Only allocate buckets_nouse when requested

It's only needed by the migrate tool - this patch adds an option to
enable allocating it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/buckets.c

@@ -2136,9 +2136,10 @@ int bch2_dev_buckets_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 					    GFP_KERNEL|__GFP_ZERO)) ||
 	    !(bucket_gens	= kvpmalloc(sizeof(struct bucket_gens) + nbuckets,
 					    GFP_KERNEL|__GFP_ZERO)) ||
-	    !(buckets_nouse	= kvpmalloc(BITS_TO_LONGS(nbuckets) *
+	    (c->opts.buckets_nouse &&
+	     !(buckets_nouse	= kvpmalloc(BITS_TO_LONGS(nbuckets) *
 					    sizeof(unsigned long),
-					    GFP_KERNEL|__GFP_ZERO)) ||
+					    GFP_KERNEL|__GFP_ZERO))) ||
 	    !init_fifo(&free[RESERVE_MOVINGGC],
 		       copygc_reserve, GFP_KERNEL) ||
 	    !init_fifo(&free[RESERVE_NONE], reserve_none, GFP_KERNEL) ||
@@ -2171,9 +2172,10 @@ int bch2_dev_buckets_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 		memcpy(bucket_gens->b,
 		       old_bucket_gens->b,
 		       n);
-		memcpy(buckets_nouse,
-		       ca->buckets_nouse,
-		       BITS_TO_LONGS(n) * sizeof(unsigned long));
+		if (buckets_nouse)
+			memcpy(buckets_nouse,
+			       ca->buckets_nouse,
+			       BITS_TO_LONGS(n) * sizeof(unsigned long));
 	}
 
 	rcu_assign_pointer(ca->buckets[0], buckets);

fs/bcachefs/opts.h

@@ -365,6 +365,11 @@ enum opt_type {
 	  NO_SB_OPT,			false,				\
 	  NULL,		"Set superblock to latest version,\n"		\
 			"allowing any new features to be used")		\
+	x(buckets_nouse,		u8,				\
+	  0,								\
+	  OPT_BOOL(),							\
+	  NO_SB_OPT,			false,				\
+	  NULL,		"Allocate the buckets_nouse bitmap")		\
 	x(project,			u8,				\
 	  OPT_INODE,							\
 	  OPT_BOOL(),							\