On 2020/12/17 18:05, Stefan Hajnoczi wrote:
> On Mon, Nov 30, 2020 at 09:11:03PM +0800, Keqian Zhu wrote:
>> @@ -1839,15 +1841,26 @@ static void dirty_memory_extend(ram_addr_t
>> old_ram_size,
>> new_blocks = g_malloc(sizeof(*new_blocks) +
>> sizeof(new_blocks->blocks[0]) *
>> new_num_blocks);
>>
>> - if (old_num_blocks) {
>> + if (cpy_num_blocks) {
>> memcpy(new_blocks->blocks, old_blocks->blocks,
>> - old_num_blocks * sizeof(old_blocks->blocks[0]));
>> + cpy_num_blocks * sizeof(old_blocks->blocks[0]));
>> }
>>
>> - for (j = old_num_blocks; j < new_num_blocks; j++) {
>> - new_blocks->blocks[j] = bitmap_new(DIRTY_MEMORY_BLOCK_SIZE);
>> + if (extend) {
>> + for (j = cpy_num_blocks; j < new_num_blocks; j++) {
>> + new_blocks->blocks[j] = bitmap_new(DIRTY_MEMORY_BLOCK_SIZE);
>> + }
>> + } else {
>> + for (j = cpy_num_blocks; j < old_num_blocks; j++) {
>> + /* We are safe to free it, for that it is out-of-use */
>> + g_free(old_blocks->blocks[j]);
>
> This looks unsafe because this code uses Read Copy Update (RCU):
>
> old_blocks = qatomic_rcu_read(&ram_list.dirty_memory[i]);
>
> Other threads may still be accessing old_blocks so we cannot modify it
> immediately. Changes need to be deferred until the next RCU period.
> g_free_rcu() needs to be used to do this.
>
Hi Stefan,
You are right. I was thinking about the VM life cycle before. We shrink the
dirty_memory
when we are removing unused ramblock. However we can not rely on this.
I also notice that "Organization into blocks allows dirty memory to grow (but
not shrink)
under RCU". Why "but not shrink"? Any thoughts?
[...]
* Organization into blocks allows dirty memory to grow (but not shrink) under
* RCU. When adding new RAMBlocks requires the dirty memory to grow, a new
* DirtyMemoryBlocks array is allocated with pointers to existing blocks kept
* the same. Other threads can safely access existing blocks while dirty
* memory is being grown. When no threads are using the old DirtyMemoryBlocks
* anymore it is freed by RCU (but the underlying blocks stay because they are
* pointed to from the new DirtyMemoryBlocks).
*/
#define DIRTY_MEMORY_BLOCK_SIZE ((ram_addr_t)256 * 1024 * 8)
typedef struct {
struct rcu_head rcu;
unsigned long *blocks[];
} DirtyMemoryBlocks;
[...]
Thanks,
Keqian
