btrfs: preallocate ulist memory for qgroup rsv

When qgroups are enabled, during data reservation, we allocate the
ulist_nodes that track the exact reserved extents with GFP_ATOMIC
unconditionally. This is unnecessary, and we can follow the model
already employed by the struct extent_state we preallocate in the non
qgroups case, which should reduce the risk of allocation failures with
GFP_ATOMIC.

Add a prealloc node to struct ulist which ulist_add will grab when it is
present, and try to allocate it before taking the tree lock while we can
still take advantage of a less strict gfp mask. The lifetime of that
node belongs to the new prealloc field, until it is used, at which point
it belongs to the ulist linked list.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-io-tree.c

@@ -4,6 +4,7 @@
 #include <trace/events/btrfs.h>
 #include "messages.h"
 #include "ctree.h"
+#include "extent_io.h"
 #include "extent-io-tree.h"
 #include "btrfs_inode.h"
 
@@ -1084,6 +1085,9 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 		 */
 		prealloc = alloc_extent_state(mask);
 	}
+	/* Optimistically preallocate the extent changeset ulist node. */
+	if (changeset)
+		extent_changeset_prealloc(changeset, mask);
 
 	spin_lock(&tree->lock);
 	if (cached_state && *cached_state) {

fs/btrfs/extent_io.h

@@ -215,6 +215,11 @@ static inline struct extent_changeset *extent_changeset_alloc(void)
 	return ret;
 }
 
+static inline void extent_changeset_prealloc(struct extent_changeset *changeset, gfp_t gfp_mask)
+{
+	ulist_prealloc(&changeset->range_changed, gfp_mask);
+}
+
 static inline void extent_changeset_release(struct extent_changeset *changeset)
 {
 	if (!changeset)

fs/btrfs/ulist.c

@@ -50,6 +50,7 @@ void ulist_init(struct ulist *ulist)
 	INIT_LIST_HEAD(&ulist->nodes);
 	ulist->root = RB_ROOT;
 	ulist->nnodes = 0;
+	ulist->prealloc = NULL;
 }
 
 /*
@@ -68,6 +69,8 @@ void ulist_release(struct ulist *ulist)
 	list_for_each_entry_safe(node, next, &ulist->nodes, list) {
 		kfree(node);
 	}
+	kfree(ulist->prealloc);
+	ulist->prealloc = NULL;
 	ulist->root = RB_ROOT;
 	INIT_LIST_HEAD(&ulist->nodes);
 }
@@ -105,6 +108,12 @@ struct ulist *ulist_alloc(gfp_t gfp_mask)
 	return ulist;
 }
 
+void ulist_prealloc(struct ulist *ulist, gfp_t gfp_mask)
+{
+	if (!ulist->prealloc)
+		ulist->prealloc = kzalloc(sizeof(*ulist->prealloc), gfp_mask);
+}
+
 /*
  * Free dynamically allocated ulist.
  *
@@ -206,9 +215,15 @@ int ulist_add_merge(struct ulist *ulist, u64 val, u64 aux,
 			*old_aux = node->aux;
 		return 0;
 	}
-	node = kmalloc(sizeof(*node), gfp_mask);
-	if (!node)
-		return -ENOMEM;
+
+	if (ulist->prealloc) {
+		node = ulist->prealloc;
+		ulist->prealloc = NULL;
+	} else {
+		node = kmalloc(sizeof(*node), gfp_mask);
+		if (!node)
+			return -ENOMEM;
+	}
 
 	node->val = val;
 	node->aux = aux;

fs/btrfs/ulist.h

@@ -41,12 +41,14 @@ struct ulist {
 
 	struct list_head nodes;
 	struct rb_root root;
+	struct ulist_node *prealloc;
 };
 
 void ulist_init(struct ulist *ulist);
 void ulist_release(struct ulist *ulist);
 void ulist_reinit(struct ulist *ulist);
 struct ulist *ulist_alloc(gfp_t gfp_mask);
+void ulist_prealloc(struct ulist *ulist, gfp_t mask);
 void ulist_free(struct ulist *ulist);
 int ulist_add(struct ulist *ulist, u64 val, u64 aux, gfp_t gfp_mask);
 int ulist_add_merge(struct ulist *ulist, u64 val, u64 aux,