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Re: [Qemu-devel] [PATCH 5/8] qcow2: Return real error in qcow2_snapshot_
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH 5/8] qcow2: Return real error in qcow2_snapshot_goto |
Date: |
Fri, 18 Nov 2011 16:08:12 +0000 |
On Thu, Nov 17, 2011 at 3:13 PM, Kevin Wolf <address@hidden> wrote:
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
> block/qcow2-snapshot.c | 50 +++++++++++++++++++++++++++++++++++++----------
> 1 files changed, 39 insertions(+), 11 deletions(-)
>
> diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
> index 066d56b..9f6647f 100644
> --- a/block/qcow2-snapshot.c
> +++ b/block/qcow2-snapshot.c
> @@ -392,17 +392,32 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const
> char *snapshot_id)
> QCowSnapshot *sn;
> int i, snapshot_index;
> int cur_l1_bytes, sn_l1_bytes;
> + int ret;
>
> + /* Search the snapshot */
> snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
> - if (snapshot_index < 0)
> + if (snapshot_index < 0) {
> return -ENOENT;
> + }
> sn = &s->snapshots[snapshot_index];
>
> - if (qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size,
> -1) < 0)
> + /* Decrease refcount of clusters of current L1 table.
> + * FIXME This is too early! */
> + ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset,
> + s->l1_size, -1);
Just following along your comments:
Here we may free clusters. Should any of the following intermediate
steps fail, we're left without a backup plan ;).
If this function fails we're in trouble. We still have the l1 table
in memory but refcounts are broken, especially if we execute
qcow2_alloc_clusters() and freed clusters get reallocated.
So if this function fails I think the image is in a dangerous state.
It may not be possible to recover data referenced by the current l1
table.
> + if (ret < 0) {
> goto fail;
> + }
>
> - if (qcow2_grow_l1_table(bs, sn->l1_size, true) < 0)
> + /*
> + * Make sure that the current L1 table is big enough to contain the whole
> + * L1 table of the snapshot. If the snapshot L1 table is smaller, the
> + * current one must be padded with zeros.
> + */
> + ret = qcow2_grow_l1_table(bs, sn->l1_size, true);
> + if (ret < 0) {
> goto fail;
> + }
>
> cur_l1_bytes = s->l1_size * sizeof(uint64_t);
> sn_l1_bytes = sn->l1_size * sizeof(uint64_t);
> @@ -411,19 +426,31 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const
> char *snapshot_id)
> memset(s->l1_table + sn->l1_size, 0, cur_l1_bytes - sn_l1_bytes);
> }
>
> - /* copy the snapshot l1 table to the current l1 table */
> - if (bdrv_pread(bs->file, sn->l1_table_offset,
> - s->l1_table, sn_l1_bytes) < 0)
> + /*
> + * Copy the snapshot L1 table to the current L1 table.
> + *
> + * Before overwriting the old current L1 table on disk, make sure to
> + * increase all refcounts for the clusters referenced by the new one.
> + */
> + ret = bdrv_pread(bs->file, sn->l1_table_offset, s->l1_table,
> sn_l1_bytes);
> + if (ret < 0) {
> goto fail;
> - if (bdrv_pwrite_sync(bs->file, s->l1_table_offset,
> - s->l1_table, cur_l1_bytes) < 0)
> + }
> +
> + ret = bdrv_pwrite(bs->file, s->l1_table_offset, s->l1_table,
> cur_l1_bytes);
Now this function does not issue an explicit bdrv_flush() anymore. Is
this really okay?
Stefan
- [Qemu-devel] [PATCH 7/8] qcow2: Fix order in qcow2_snapshot_delete, (continued)
- [Qemu-devel] [PATCH 7/8] qcow2: Fix order in qcow2_snapshot_delete, Kevin Wolf, 2011/11/17
- [Qemu-devel] [PATCH 6/8] qcow2: Fix order of refcount updates in qcow2_snapshot_goto, Kevin Wolf, 2011/11/17
- [Qemu-devel] [PATCH 2/8] qcow2: Return real error code in qcow2_write_snapshots, Kevin Wolf, 2011/11/17
- [Qemu-devel] [PATCH 8/8] qcow2: Fix error path in qcow2_snapshot_load_tmp, Kevin Wolf, 2011/11/17
- [Qemu-devel] [PATCH 5/8] qcow2: Return real error in qcow2_snapshot_goto, Kevin Wolf, 2011/11/17
- Re: [Qemu-devel] [PATCH 5/8] qcow2: Return real error in qcow2_snapshot_goto,
Stefan Hajnoczi <=
- Re: [Qemu-devel] [PATCH 0/8] qcow2: Fix error paths for internal snapshots, Stefan Hajnoczi, 2011/11/18