btrfs: send: index backref cache by node number instead of by sector number

We now have a nodesize_bits member in fs_info so we can index an extent
buffer in the backref cache by node number instead of by sector number.
While this allows for a denser index space with the possibility of using
less maple tree nodes, in practice it's unlikely to hit such benefits
since we currently limit the maximum number of keys in the cache to 128,
so unless all extent buffers are contiguous we are unlikely to see a
memory usage reduction in the backing maple tree due to less nodes.
Nevertheless it doesn't cost anything to index by node number and it's
more logical.

Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/send.c

@@ -1388,7 +1388,7 @@ static bool lookup_backref_cache(u64 leaf_bytenr, void *ctx,
 	struct backref_ctx *bctx = ctx;
 	struct send_ctx *sctx = bctx->sctx;
 	struct btrfs_fs_info *fs_info = sctx->send_root->fs_info;
-	const u64 key = leaf_bytenr >> fs_info->sectorsize_bits;
+	const u64 key = leaf_bytenr >> fs_info->nodesize_bits;
 	struct btrfs_lru_cache_entry *raw_entry;
 	struct backref_cache_entry *entry;
 
@@ -1443,7 +1443,7 @@ static void store_backref_cache(u64 leaf_bytenr, const struct ulist *root_ids,
 	if (!new_entry)
 		return;
 
-	new_entry->entry.key = leaf_bytenr >> fs_info->sectorsize_bits;
+	new_entry->entry.key = leaf_bytenr >> fs_info->nodesize_bits;
 	new_entry->entry.gen = 0;
 	new_entry->num_roots = 0;
 	ULIST_ITER_INIT(&uiter);