[PATCH v2 02/16] writeback: add support to initialize and free multiple writeback ctxs

Kundan Kumar
2025-10-14T12:10:26Z

Introduce a new macro for_each_bdi_wb_ctx to iterate over multiple
writeback ctxs. Added logic for allocation, init, free, registration
and unregistration of multiple writeback contexts within a bdi.

Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
---
include/linux/backing-dev.h | 4 ++
mm/backing-dev.c | 81 +++++++++++++++++++++++++++----------
2 files changed, 63 insertions(+), 22 deletions(-)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 92674543ac8a..951ab5497500 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -148,6 +148,10 @@ static inline bool mapping_can_writeback(struct address_space *mapping)
return inode_to_bdi(mapping->host)->capabilities & BDI_CAP_WRITEBACK;
}

+#define for_each_bdi_wb_ctx(bdi, wbctx) \
+ for (int __i = 0; __i < (bdi)->nr_wb_ctx \
+ && ((wbctx) = (bdi)->wb_ctx[__i]) != NULL; __i++)
+
static inline struct bdi_writeback_ctx *
fetch_bdi_writeback_ctx(struct inode *inode)
{
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 8b7125349f6c..47196d326e16 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -835,17 +835,20 @@ struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,

static int cgwb_bdi_init(struct backing_dev_info *bdi)
{
- int ret;
- struct bdi_writeback_ctx *bdi_wb_ctx = bdi->wb_ctx[0];
+ int ret = 0;
+ struct bdi_writeback_ctx *bdi_wb_ctx;

- INIT_RADIX_TREE(&bdi_wb_ctx->cgwb_tree, GFP_ATOMIC);
- mutex_init(&bdi->cgwb_release_mutex);
- init_rwsem(&bdi_wb_ctx->wb_switch_rwsem);
+ for_each_bdi_wb_ctx(bdi, bdi_wb_ctx) {
+ INIT_RADIX_TREE(&bdi_wb_ctx->cgwb_tree, GFP_ATOMIC);
+ mutex_init(&bdi->cgwb_release_mutex);
+ init_rwsem(&bdi_wb_ctx->wb_switch_rwsem);

- ret = wb_init(&bdi_wb_ctx->wb, bdi, GFP_KERNEL);
- if (!ret) {
- bdi_wb_ctx->wb.memcg_css = &root_mem_cgroup->css;
- bdi_wb_ctx->wb.blkcg_css = blkcg_root_css;
+ ret = wb_init(&bdi_wb_ctx->wb, bdi, GFP_KERNEL);
+ if (!ret) {
+ bdi_wb_ctx->wb.memcg_css = &root_mem_cgroup->css;
+ bdi_wb_ctx->wb.blkcg_css = blkcg_root_css;
+ } else
+ return ret;
}
return ret;
}
@@ -992,7 +995,16 @@ subsys_initcall(cgwb_init);

static int cgwb_bdi_init(struct backing_dev_info *bdi)
{
- return wb_init(&bdi->wb_ctx[0]->wb, bdi, GFP_KERNEL);
+ struct bdi_writeback_ctx *bdi_wb_ctx;
+
+ for_each_bdi_wb_ctx(bdi, bdi_wb_ctx) {
+ int ret;
+
+ ret = wb_init(&bdi_wb_ctx->wb, bdi, GFP_KERNEL);
+ if (ret)
+ return ret;
+ }
+ return 0;
}

static void cgwb_bdi_unregister(struct backing_dev_info *bdi,
@@ -1026,10 +1038,19 @@ int bdi_init(struct backing_dev_info *bdi)
sizeof(struct bdi_writeback_ctx *),
GFP_KERNEL);
INIT_LIST_HEAD(&bdi->bdi_list);
- bdi->wb_ctx[0] = (struct bdi_writeback_ctx *)
- kzalloc(sizeof(struct bdi_writeback_ctx), GFP_KERNEL);
- INIT_LIST_HEAD(&bdi->wb_ctx[0]->wb_list);
- init_waitqueue_head(&bdi->wb_ctx[0]->wb_waitq);
+ for (int i = 0; i < bdi->nr_wb_ctx; i++) {
+ bdi->wb_ctx[i] = (struct bdi_writeback_ctx *)
+ kzalloc(sizeof(struct bdi_writeback_ctx), GFP_KERNEL);
+ if (!bdi->wb_ctx[i]) {
+ pr_err("Failed to allocate %d", i);
+ while (--i >= 0)
+ kfree(bdi->wb_ctx[i]);
+ kfree(bdi->wb_ctx);
+ return -ENOMEM;
+ }
+ INIT_LIST_HEAD(&bdi->wb_ctx[i]->wb_list);
+ init_waitqueue_head(&bdi->wb_ctx[i]->wb_waitq);
+ }
bdi->last_bdp_sleep = jiffies;

return cgwb_bdi_init(bdi);
@@ -1038,13 +1059,16 @@ int bdi_init(struct backing_dev_info *bdi)
struct backing_dev_info *bdi_alloc(int node_id)
{
struct backing_dev_info *bdi;
+ struct bdi_writeback_ctx *bdi_wb_ctx;

bdi = kzalloc_node(sizeof(*bdi), GFP_KERNEL, node_id);
if (!bdi)
return NULL;

if (bdi_init(bdi)) {
- kfree(bdi->wb_ctx[0]);
+ for_each_bdi_wb_ctx(bdi, bdi_wb_ctx) {
+ kfree(bdi_wb_ctx);
+ }
kfree(bdi->wb_ctx);
kfree(bdi);
return NULL;
@@ -1109,6 +1133,7 @@ int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list args)
{
struct device *dev;
struct rb_node *parent, **p;
+ struct bdi_writeback_ctx *bdi_wb_ctx;

if (bdi->dev) /* The driver needs to use separate queues per device */
return 0;
@@ -1118,8 +1143,11 @@ int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list args)
if (IS_ERR(dev))
return PTR_ERR(dev);

- cgwb_bdi_register(bdi, bdi->wb_ctx[0]);
- set_bit(WB_registered, &bdi->wb_ctx[0]->wb.state);
+ for_each_bdi_wb_ctx(bdi, bdi_wb_ctx) {
+ cgwb_bdi_register(bdi, bdi_wb_ctx);
+ set_bit(WB_registered, &bdi_wb_ctx->wb.state);
+ }
+
bdi->dev = dev;

bdi_debug_register(bdi, dev_name(dev));
@@ -1174,12 +1202,17 @@ static void bdi_remove_from_list(struct backing_dev_info *bdi)

void bdi_unregister(struct backing_dev_info *bdi)
{
+ struct bdi_writeback_ctx *bdi_wb_ctx;
+
timer_delete_sync(&bdi->laptop_mode_wb_timer);

/* make sure nobody finds us on the bdi_list anymore */
bdi_remove_from_list(bdi);
- wb_shutdown(&bdi->wb_ctx[0]->wb);
- cgwb_bdi_unregister(bdi, bdi->wb_ctx[0]);
+
+ for_each_bdi_wb_ctx(bdi, bdi_wb_ctx) {
+ wb_shutdown(&bdi_wb_ctx->wb);
+ cgwb_bdi_unregister(bdi, bdi_wb_ctx);
+ }

/*
* If this BDI's min ratio has been set, use bdi_set_min_ratio() to
@@ -1205,11 +1238,15 @@ static void release_bdi(struct kref *ref)
{
struct backing_dev_info *bdi =
container_of(ref, struct backing_dev_info, refcnt);
+ struct bdi_writeback_ctx *bdi_wb_ctx;

WARN_ON_ONCE(bdi->dev);
- WARN_ON_ONCE(test_bit(WB_registered, &bdi->wb_ctx[0]->wb.state));
- wb_exit(&bdi->wb_ctx[0]->wb);
- kfree(bdi->wb_ctx[0]);
+
+ for_each_bdi_wb_ctx(bdi, bdi_wb_ctx) {
+ WARN_ON_ONCE(test_bit(WB_registered, &bdi_wb_ctx->wb.state));
+ wb_exit(&bdi_wb_ctx->wb);
+ kfree(bdi_wb_ctx);
+ }
kfree(bdi->wb_ctx);
kfree(bdi);
}
--
2.25.1