bcachefs: BCH_SB_RESERVE_BYTES

Add an option, gc_reserve_bytes, to set the copygc reserve as a size
instead of a percent
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc.c

@@ -1711,7 +1711,7 @@ void bch2_alloc_sectors_done(struct bch_fs *c, struct write_point *wp)
 void bch2_recalc_capacity(struct bch_fs *c)
 {
 	struct bch_dev *ca;
-	u64 capacity = 0, reserved_sectors = 0;
+	u64 capacity = 0, reserved_sectors = 0, gc_reserve;
 	unsigned long ra_pages = 0;
 	unsigned i, j;
 
@@ -1726,7 +1726,7 @@ void bch2_recalc_capacity(struct bch_fs *c)
 	bch2_set_ra_pages(c, ra_pages);
 
 	for_each_rw_member(ca, c, i) {
-		u64 dev_capacity, dev_reserve = 0;
+		u64 dev_reserve = 0;
 
 		/*
 		 * We need to reserve buckets (from the number
@@ -1758,25 +1758,21 @@ void bch2_recalc_capacity(struct bch_fs *c)
 
 		dev_reserve *= ca->mi.bucket_size;
 
-		dev_reserve *= 2;
+		ca->copygc_threshold = dev_reserve;
 
-		dev_capacity = bucket_to_sector(ca, ca->mi.nbuckets -
-						ca->mi.first_bucket);
+		capacity += bucket_to_sector(ca, ca->mi.nbuckets -
+					     ca->mi.first_bucket);
 
-		ca->copygc_threshold =
-			max(div64_u64(dev_capacity *
-				      c->opts.gc_reserve_percent, 100),
-			    dev_reserve) / 2;
-
-		capacity += dev_capacity;
-		reserved_sectors += dev_reserve;
+		reserved_sectors += dev_reserve * 2;
 	}
 
-	reserved_sectors = max(div64_u64(capacity *
-					 c->opts.gc_reserve_percent, 100),
-			       reserved_sectors);
+	gc_reserve = c->opts.gc_reserve_bytes
+		? c->opts.gc_reserve_bytes >> 9
+		: div64_u64(capacity * c->opts.gc_reserve_percent, 100);
+
+	reserved_sectors = max(gc_reserve, reserved_sectors);
 
-	BUG_ON(reserved_sectors > capacity);
+	reserved_sectors = min(reserved_sectors, capacity);
 
 	c->capacity = capacity - reserved_sectors;
 

fs/bcachefs/bcachefs_format.h

@@ -1221,6 +1221,7 @@ LE64_BITMASK(BCH_SB_BACKGROUND_TARGET,	struct bch_sb, flags[1], 52, 64);
 
 LE64_BITMASK(BCH_SB_BACKGROUND_COMPRESSION_TYPE,
 					struct bch_sb, flags[2],  0,  4);
+LE64_BITMASK(BCH_SB_GC_RESERVE_BYTES,	struct bch_sb, flags[2],  4, 64);
 
 /* Features: */
 enum bch_sb_features {

fs/bcachefs/opts.h

@@ -114,9 +114,12 @@ enum opt_type {
 	BCH_OPT(inodes_32bit,		u8,	OPT_RUNTIME,		\
 		OPT_BOOL(),						\
 		BCH_SB_INODE_32BIT,		false)			\
-	BCH_OPT(gc_reserve_percent,	u8,	OPT_MOUNT,		\
+	BCH_OPT(gc_reserve_percent,	u8,	OPT_RUNTIME,		\
 		OPT_UINT(5, 21),					\
 		BCH_SB_GC_RESERVE,		8)			\
+	BCH_OPT(gc_reserve_bytes,	u64,	OPT_RUNTIME,		\
+		OPT_UINT(0, U64_MAX),					\
+		BCH_SB_GC_RESERVE_BYTES,	0)			\
 	BCH_OPT(root_reserve_percent,	u8,	OPT_MOUNT,		\
 		OPT_UINT(0, 100),					\
 		BCH_SB_ROOT_RESERVE,		0)			\