On Wed, Nov 19, 2025 at 04:14:51PM -0800, Gustavo Luiz Duarte wrote:
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
Reviewed-by: Breno Leitao leitao@debian.org