writeback: introduce cgwb_v1 boot param

So far writeback control is supported for cgroup v1 interface. However
it also has some restrictions, so introduce a new kernel boot parameter
to control the behavior which is disabled by default. Users can enable
the writeback control for cgroup v1 with the command line "cgwb_v1".
Signed-off-by: NJiufei Xue <jiufei.xue@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -496,6 +496,9 @@
 			nosocket -- Disable socket memory accounting.
 			nokmem -- Disable kernel memory accounting.
 
+	cgwb_v1		Enable writeback control for cgroup for cgroup v1
+			interface.
+
 	checkreqprot	[SELINUX] Set initial checkreqprot flag value.
 			Format: { "0" | "1" }
 			See security/selinux/Kconfig help text.

include/linux/backing-dev.h

@@ -233,6 +233,21 @@ void wb_memcg_offline(struct mem_cgroup *memcg);
 void wb_blkcg_offline(struct blkcg *blkcg);
 int inode_congested(struct inode *inode, int cong_bits);
 
+extern bool cgwb_v1;
+
+static inline bool memcg_blkcg_on_dfl(void)
+{
+	return cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
+	       cgroup_subsys_on_dfl(io_cgrp_subsys);
+}
+
+static inline bool cgroup_writeback_support_v1(void)
+{
+	return cgwb_v1 &&
+	       !cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
+	       !cgroup_subsys_on_dfl(io_cgrp_subsys);
+}
+
 /**
  * inode_cgwb_enabled - test whether cgroup writeback is enabled on an inode
  * @inode: inode of interest
@@ -248,7 +263,9 @@ static inline bool inode_cgwb_enabled(struct inode *inode)
 {
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 
-	return bdi_cap_account_dirty(bdi) &&
+	return (memcg_blkcg_on_dfl() ||
+		cgroup_writeback_support_v1()) &&
+		bdi_cap_account_dirty(bdi) &&
 		(bdi->capabilities & BDI_CAP_CGROUP_WRITEBACK) &&
 		(inode->i_sb->s_iflags & SB_I_CGROUPWB);
 }

mm/backing-dev.c

@@ -49,6 +49,9 @@ static void bdi_debug_init(void)
 	if (!bdi_debug_root)
 		return;
 
+	if (!cgwb_v1)
+		return;
+
 	memcg_blkcg_file = debugfs_create_file("bdi_wb_link", 0444, bdi_debug_root,
 			NULL, &memcg_blkcg_debug_fops);
 #endif
@@ -455,6 +458,9 @@ int allocate_memcg_blkcg_links(int count, struct list_head *tmp_links)
 	struct memcg_blkcg_link *link;
 	int i;
 
+	if (!cgwb_v1)
+		return 0;
+
 	for (i = 0; i < count; i++) {
 		link = kzalloc(sizeof(*link), GFP_KERNEL);
 		if (!link) {
@@ -482,6 +488,9 @@ void insert_memcg_blkcg_link(struct cgroup_subsys *ss,
 	struct cgroup_subsys_state *memcg_css;
 	int err;
 
+	if (!cgwb_v1)
+		return;
+
 	if (ss->id != io_cgrp_id && ss->id != memory_cgrp_id)
 		return;
 
@@ -567,6 +576,9 @@ static void delete_blkcg_link(struct cgroup_subsys_state *blkcg_css)
 void delete_memcg_blkcg_link(struct cgroup_subsys *ss,
 			     struct cgroup_subsys_state *css)
 {
+	if (!cgwb_v1)
+		return;
+
 	if (ss->id != io_cgrp_id && ss->id != memory_cgrp_id)
 		return;
 

mm/memcontrol.c

@@ -468,7 +468,8 @@ struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
 
 	memcg = page->mem_cgroup;
 
-	if (!memcg)
+	if (!memcg ||
+	   (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgwb_v1))
 		memcg = root_mem_cgroup;
 
 	return &memcg->css;
@@ -6369,6 +6370,16 @@ static int __init cgroup_memory(char *s)
 }
 __setup("cgroup.memory=", cgroup_memory);
 
+bool cgwb_v1 = false;
+
+static int __init enable_cgroup_writeback_v1(char *s)
+{
+	cgwb_v1 = true;
+
+	return 0;
+}
+__setup("cgwb_v1", enable_cgroup_writeback_v1);
+
 /*
  * subsys_initcall() for memory controller.
  *