[PATCH 8/9] kvm tools, qcow: Delayed L2 table writeout

Pekka Enberg
2011-07-09T13:03:04Z

This patch delays writeout for new L2 tables like we do for L1 tables. If a L2
table has non-allocated clusters, we mark that in the in-memory L2 table but
don't actually write it to disk until the L2 table is thrown out of LRU cache
or when qcow_disk_flush() is called. That makes writes to new clusters volatile
before VIRTIO_BLK_T_FLUSH is issued without corrupting the QCOW image on I/O
error.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
---
tools/kvm/disk/qcow.c | 66 +++++++++++++++++++++++++++++--------------------
1 files changed, 39 insertions(+), 27 deletions(-)

diff --git a/tools/kvm/disk/qcow.c b/tools/kvm/disk/qcow.c
index c851e7f..b71762f 100644
--- a/tools/kvm/disk/qcow.c
+++ b/tools/kvm/disk/qcow.c
@@ -88,6 +88,16 @@ static void free_cache(struct qcow *q)
}
}

+static int qcow_l2_cache_write(struct qcow *q, struct qcow_l2_table *c)
+{
+ struct qcow_header *header = q->header;
+ u64 size;
+
+ size = 1 << header->l2_bits;
+
+ return pwrite_in_full(q->fd, c->table, size * sizeof(u64), c->offset);
+}
+
static int cache_table(struct qcow *q, struct qcow_l2_table *c)
{
struct rb_root *r = &q->root;
@@ -100,6 +110,9 @@ static int cache_table(struct qcow *q, struct qcow_l2_table *c)
*/
lru = list_first_entry(&q->lru_list, struct qcow_l2_table, list);

+ if (qcow_l2_cache_write(q, lru) < 0)
+ goto error;
+
/* Remove the node from the cache */
rb_erase(&lru->node, r);
list_del_init(&lru->list);
@@ -361,7 +374,6 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
struct qcow_header *header = q->header;
struct qcow_table *table = &q->table;
struct qcow_l2_table *l2t;
- bool update_meta;
u64 clust_start;
u64 clust_off;
u64 clust_sz;
@@ -371,7 +383,6 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
u64 l2t_sz;
u64 f_sz;
u64 len;
- u64 t;

l2t = NULL;
l2t_sz = 1 << header->l2_bits;
@@ -434,31 +445,16 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src

clust_start = be64_to_cpu(l2t->table[l2t_idx]) & ~header->oflag_mask;
if (!clust_start) {
- clust_start = ALIGN(f_sz, clust_sz);
- update_meta = true;
- } else
- update_meta = false;
-
- /* Write actual data */
- if (pwrite_in_full(q->fd, buf, len, clust_start + clust_off) < 0)
- goto error;
-
- if (update_meta) {
- t = cpu_to_be64(clust_start);
- if (qcow_pwrite_sync(q->fd, &t, sizeof(t), l2t_off + l2t_idx * sizeof(u64)) < 0) {
- /* Restore the file to consistent state */
- if (ftruncate(q->fd, f_sz) < 0)
- goto error;
-
- goto error;
- }
-
- /* Update the cached level2 entry */
- l2t->table[l2t_idx] = cpu_to_be64(clust_start);
+ clust_start = ALIGN(f_sz, clust_sz);
+ l2t->table[l2t_idx] = cpu_to_be64(clust_start);
}

mutex_unlock(&q->mutex);

+ /* Write actual data */
+ if (pwrite_in_full(q->fd, buf, len, clust_start + clust_off) < 0)
+ return -1;
+
return len;

free_cache:
@@ -508,18 +504,34 @@ static int qcow_disk_flush(struct disk_image *disk)
{
struct qcow *q = disk->priv;
struct qcow_header *header;
+ struct list_head *pos, *n;
struct qcow_table *table;

- if (fdatasync(disk->fd) < 0)
- return -1;
-
header = q->header;
table = &q->table;

+ mutex_lock(&q->mutex);
+
+ list_for_each_safe(pos, n, &q->lru_list) {
+ struct qcow_l2_table *c = list_entry(pos, struct qcow_l2_table, list);
+
+ if (qcow_l2_cache_write(q, c) < 0)
+ goto error_unlock;
+ }
+
+ if (fdatasync(disk->fd) < 0)
+ goto error_unlock;
+
if (pwrite_in_full(disk->fd, table->l1_table, table->table_size * sizeof(u64), header->l1_table_offset) < 0)
- return -1;
+ goto error_unlock;
+
+ mutex_unlock(&q->mutex);

return fsync(disk->fd);
+
+error_unlock:
+ mutex_unlock(&q->mutex);
+ return -1;
}

static int qcow_disk_close(struct disk_image *disk)
--
1.7.0.4