The userdata buffer in struct netconsole_target is currently statically allocated with a size of MAX_USERDATA_ITEMS * MAX_EXTRADATA_ENTRY_LEN (16 * 256 = 4096 bytes). This wastes memory when userdata entries are not used or when only a few entries are configured, which is common in typical usage scenarios. It also forces us to keep MAX_USERDATA_ITEMS small to limit the memory wasted.
Change the userdata buffer from a static array to a dynamically allocated pointer. The buffer is now allocated on-demand in update_userdata() whenever userdata entries are added, modified, or removed via configfs. The implementation calculates the exact size needed for all current userdata entries, allocates a new buffer of that size, formats the entries into it, and atomically swaps it with the old buffer.
This approach provides several benefits: - Memory efficiency: Targets with no userdata use zero bytes instead of 4KB, and targets with userdata only allocate what they need; - Scalability: Makes it practical to increase MAX_USERDATA_ITEMS to a much larger value without imposing a fixed memory cost on every target; - No hot-path overhead: Allocation occurs during configuration (write to configfs), not during message transmission
If memory allocation fails during userdata update, -ENOMEM is returned to userspace through the configfs attribute write operation.
The sysdata buffer remains statically allocated since it has a smaller fixed size (MAX_SYSDATA_ITEMS * MAX_EXTRADATA_ENTRY_LEN = 4 * 256 = 1024 bytes) and its content length is less predictable.
Signed-off-by: Gustavo Luiz Duarte gustavold@gmail.com --- drivers/net/netconsole.c | 100 +++++++++++++++++++++++++++++++++-------------- 1 file changed, 70 insertions(+), 30 deletions(-)
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c index 1bd811714322..0b350f82d915 100644 --- a/drivers/net/netconsole.c +++ b/drivers/net/netconsole.c @@ -155,7 +155,7 @@ struct netconsole_target { #ifdef CONFIG_NETCONSOLE_DYNAMIC struct config_group group; struct config_group userdata_group; - char userdata[MAX_EXTRADATA_ENTRY_LEN * MAX_USERDATA_ITEMS]; + char *userdata; size_t userdata_length; char sysdata[MAX_EXTRADATA_ENTRY_LEN * MAX_SYSDATA_ITEMS];
@@ -875,45 +875,77 @@ static ssize_t userdatum_value_show(struct config_item *item, char *buf) return sysfs_emit(buf, "%s\n", &(to_userdatum(item)->value[0])); }
-static void update_userdata(struct netconsole_target *nt) +/* Navigate configfs and calculate the lentgh of the formatted string + * representing userdata. + * Must be called holding netconsole_subsys.su_mutex + */ +static int calc_userdata_len(struct netconsole_target *nt) { + struct userdatum *udm_item; + struct config_item *item; struct list_head *entry; - int child_count = 0; - unsigned long flags; + int len = 0;
- spin_lock_irqsave(&target_list_lock, flags); + list_for_each(entry, &nt->userdata_group.cg_children) { + item = container_of(entry, struct config_item, ci_entry); + udm_item = to_userdatum(item); + /* Skip userdata with no value set */ + if (udm_item->value[0]) { + len += snprintf(NULL, 0, " %s=%s\n", item->ci_name, + udm_item->value); + } + } + return len; +}
- /* Clear the current string in case the last userdatum was deleted */ - nt->userdata_length = 0; - nt->userdata[0] = 0; +static int update_userdata(struct netconsole_target *nt) +{ + struct userdatum *udm_item; + struct config_item *item; + struct list_head *entry; + char *old_buf = NULL; + char *new_buf = NULL; + unsigned long flags; + int offset = 0; + int len;
- list_for_each(entry, &nt->userdata_group.cg_children) { - struct userdatum *udm_item; - struct config_item *item; + /* Calculate required buffer size */ + len = calc_userdata_len(nt);
- if (child_count >= MAX_USERDATA_ITEMS) { - spin_unlock_irqrestore(&target_list_lock, flags); - WARN_ON_ONCE(1); - return; - } - child_count++; + if (WARN_ON_ONCE(len > MAX_EXTRADATA_ENTRY_LEN * MAX_USERDATA_ITEMS)) + return -ENOSPC; + + /* Allocate new buffer */ + if (len) { + new_buf = kmalloc(len + 1, GFP_KERNEL); + if (!new_buf) + return -ENOMEM; + }
+ /* Write userdata to new buffer */ + list_for_each(entry, &nt->userdata_group.cg_children) { item = container_of(entry, struct config_item, ci_entry); udm_item = to_userdatum(item); - /* Skip userdata with no value set */ - if (strnlen(udm_item->value, MAX_EXTRADATA_VALUE_LEN) == 0) - continue; - - /* This doesn't overflow userdata since it will write - * one entry length (1/MAX_USERDATA_ITEMS long), entry count is - * checked to not exceed MAX items with child_count above - */ - nt->userdata_length += scnprintf(&nt->userdata[nt->userdata_length], - MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n", - item->ci_name, udm_item->value); + if (udm_item->value[0]) { + offset += scnprintf(&new_buf[offset], len + 1 - offset, + " %s=%s\n", item->ci_name, + udm_item->value); + } } + + WARN_ON_ONCE(offset != len); + + /* Switch to new buffer and free old buffer */ + spin_lock_irqsave(&target_list_lock, flags); + old_buf = nt->userdata; + nt->userdata = new_buf; + nt->userdata_length = offset; spin_unlock_irqrestore(&target_list_lock, flags); + + kfree(old_buf); + + return 0; }
static ssize_t userdatum_value_store(struct config_item *item, const char *buf, @@ -937,7 +969,9 @@ static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
ud = to_userdata(item->ci_parent); nt = userdata_to_target(ud); - update_userdata(nt); + ret = update_userdata(nt); + if (ret < 0) + goto out_unlock; ret = count; out_unlock: mutex_unlock(&dynamic_netconsole_mutex); @@ -1193,7 +1227,10 @@ static struct configfs_attribute *netconsole_target_attrs[] = {
static void netconsole_target_release(struct config_item *item) { - kfree(to_target(item)); + struct netconsole_target *nt = to_target(item); + + kfree(nt->userdata); + kfree(nt); }
static struct configfs_item_operations netconsole_target_item_ops = { @@ -1874,6 +1911,9 @@ static struct netconsole_target *alloc_param_target(char *target_config, static void free_param_target(struct netconsole_target *nt) { netpoll_cleanup(&nt->np); +#ifdef CONFIG_NETCONSOLE_DYNAMIC + kfree(nt->userdata); +#endif kfree(nt); }