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memory: RCU ram_list.dirty_memory[] for safe RAM hotplug

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Although accesses to ram_list.dirty_memory[] use atomics so multiple
threads can safely dirty the bitmap, the data structure is not fully
thread-safe yet.

This patch handles the RAM hotplug case where ram_list.dirty_memory[] is
grown.  ram_list.dirty_memory[] is change from a regular bitmap to an
RCU array of pointers to fixed-size bitmap blocks.  Threads can continue
accessing bitmap blocks while the array is being extended.  See the
comments in the code for an in-depth explanation of struct
DirtyMemoryBlocks.

I have tested that live migration with virtio-blk dataplane works.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1453728801-5398-2-git-send-email-stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2016-01-25 13:33:20 +00:00 • committed by Paolo Bonzini
parent 8bafcb2164
commit 5b82b703b6
3 changed files with 225 additions and 43 deletions
Download patch file Download diff file Expand all files Collapse all files

75
exec.c
View file

@ -980,8 +980,9 @@ bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
ram_addr_t length,
unsigned client)
{
DirtyMemoryBlocks *blocks;
unsigned long end, page;
bool dirty;
bool dirty = false;
if (length == 0) {
return false;
@ -989,8 +990,22 @@ bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
page = start >> TARGET_PAGE_BITS;
dirty = bitmap_test_and_clear_atomic(ram_list.dirty_memory[client],
page, end - page);
rcu_read_lock();
blocks = atomic_rcu_read(&ram_list.dirty_memory[client]);
while (page < end) {
unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
unsigned long num = MIN(end - page, DIRTY_MEMORY_BLOCK_SIZE - offset);
dirty |= bitmap_test_and_clear_atomic(blocks->blocks[idx],
offset, num);
page += num;
}
rcu_read_unlock();
if (dirty && tcg_enabled()) {
tlb_reset_dirty_range_all(start, length);
@ -1504,6 +1519,47 @@ int qemu_ram_resize(ram_addr_t base, ram_addr_t newsize, Error **errp)
return 0;
}
/* Called with ram_list.mutex held */
static void dirty_memory_extend(ram_addr_t old_ram_size,
ram_addr_t new_ram_size)
{
ram_addr_t old_num_blocks = DIV_ROUND_UP(old_ram_size,
DIRTY_MEMORY_BLOCK_SIZE);
ram_addr_t new_num_blocks = DIV_ROUND_UP(new_ram_size,
DIRTY_MEMORY_BLOCK_SIZE);
int i;
/* Only need to extend if block count increased */
if (new_num_blocks <= old_num_blocks) {
return;
}
for (i = 0; i < DIRTY_MEMORY_NUM; i++) {
DirtyMemoryBlocks *old_blocks;
DirtyMemoryBlocks *new_blocks;
int j;
old_blocks = atomic_rcu_read(&ram_list.dirty_memory[i]);
new_blocks = g_malloc(sizeof(*new_blocks) +
sizeof(new_blocks->blocks[0]) * new_num_blocks);
if (old_num_blocks) {
memcpy(new_blocks->blocks, old_blocks->blocks,
old_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);
}
atomic_rcu_set(&ram_list.dirty_memory[i], new_blocks);
if (old_blocks) {
g_free_rcu(old_blocks, rcu);
}
}
}
static ram_addr_t ram_block_add(RAMBlock *new_block, Error **errp)
{
RAMBlock *block;
@ -1543,6 +1599,7 @@ static ram_addr_t ram_block_add(RAMBlock *new_block, Error **errp)
(new_block->offset + new_block->max_length) >> TARGET_PAGE_BITS);
if (new_ram_size > old_ram_size) {
migration_bitmap_extend(old_ram_size, new_ram_size);
dirty_memory_extend(old_ram_size, new_ram_size);
}
/* Keep the list sorted from biggest to smallest block. Unlike QTAILQ,
* QLIST (which has an RCU-friendly variant) does not have insertion at
@ -1568,18 +1625,6 @@ static ram_addr_t ram_block_add(RAMBlock *new_block, Error **errp)
ram_list.version++;
qemu_mutex_unlock_ramlist();
new_ram_size = last_ram_offset() >> TARGET_PAGE_BITS;
if (new_ram_size > old_ram_size) {
int i;
/* ram_list.dirty_memory[] is protected by the iothread lock. */
for (i = 0; i < DIRTY_MEMORY_NUM; i++) {
ram_list.dirty_memory[i] =
bitmap_zero_extend(ram_list.dirty_memory[i],
old_ram_size, new_ram_size);
}
}
cpu_physical_memory_set_dirty_range(new_block->offset,
new_block->used_length,
DIRTY_CLIENTS_ALL);