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Re: [Qemu-devel] [PATCH v2 2/2] qcow2: truncate the tail of the image fi

From: Pavel Butsykin
Subject: Re: [Qemu-devel] [PATCH v2 2/2] qcow2: truncate the tail of the image file after shrinking the image
Date: Thu, 28 Sep 2017 11:57:07 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 
On 27.09.2017 19:36, Max Reitz wrote:

On 2017-09-27 18:27, Pavel Butsykin wrote:

On 27.09.2017 19:00, Max Reitz wrote:

On 2017-09-22 11:39, Pavel Butsykin wrote:

Now after shrinking the image, at the end of the image file, there
might be a
tail that probably will never be used. So we can find the last used
cluster and
cut the tail.

Signed-off-by: Pavel Butsykin <address@hidden>
---
   block/qcow2-refcount.c | 22 ++++++++++++++++++++++
   block/qcow2.c          | 23 +++++++++++++++++++++++
   block/qcow2.h          |  1 +
   3 files changed, 46 insertions(+)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 88d5a3f1ad..aa3fd6cf17 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -3181,3 +3181,25 @@ out:
       g_free(reftable_tmp);
       return ret;
   }
+
+int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int64_t i;
+
+    for (i = size_to_clusters(s, size) - 1; i >= 0; i--) {
+        uint64_t refcount;
+        int ret = qcow2_get_refcount(bs, i, &refcount);
+        if (ret < 0) {
+            fprintf(stderr, "Can't get refcount for cluster %"
PRId64 ": %s\n",
+                    i, strerror(-ret));
+            return ret;
+        }
+        if (refcount > 0) {
+            return i;
+        }
+    }
+    qcow2_signal_corruption(bs, true, -1, -1,
+                            "There are no references in the refcount
table.");
+    return -EIO;
+}
diff --git a/block/qcow2.c b/block/qcow2.c
index 8a4311d338..8dfb5393a7 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3106,6 +3106,7 @@ static int qcow2_truncate(BlockDriverState *bs,
int64_t offset,
       new_l1_size = size_to_l1(s, offset);
         if (offset < old_length) {
+        int64_t last_cluster, old_file_size;
           if (prealloc != PREALLOC_MODE_OFF) {
               error_setg(errp,
                          "Preallocation can't be used for shrinking
an image");
@@ -3134,6 +3135,28 @@ static int qcow2_truncate(BlockDriverState
*bs, int64_t offset,
                                "Failed to discard unused refblocks");
               return ret;
           }
+
+        old_file_size = bdrv_getlength(bs->file->bs);
+        if (old_file_size < 0) {
+            error_setg_errno(errp, -old_file_size,
+                             "Failed to inquire current file length");
+            return old_file_size;
+        }
+        last_cluster = qcow2_get_last_cluster(bs, old_file_size);
+        if (last_cluster < 0) {
+            error_setg_errno(errp, -last_cluster,
+                             "Failed to find the last cluster");
+            return last_cluster;
+        }


My idea was rather that you just wouldn't truncate the image file if
something fails here.  So in any of these new cases where you currently
just report the whole truncate operation as having failed, you could
just emit a warning and not do the truncation of bs->file.


I'm not sure that's right. If the qcow2_get_last_cluster() returned an
error, probably with the image was a problem.. can we continue to work
with the image without risking to damage it even more? if something bad
happened with the reftable we usually mark the image as corrupted, it's
the same thing.


Well, the only thing that's left to do is to write the new size into the
image header, so I think that should work just fine...


Yes, but what difference will update the size in the header or not, if
the reftable was corrupted. A much more important point here is that the
qcow2_truncate() should return an error and the caller must stop the
work.


I won't disagree that bdrv_getlength() or qcow2_get_last_cluster()
failing may be reasons to stop truncation (although I don't think they
necessarily are at this point).

But I could well imagine that the below bdrv_truncate() of bs->file
fails for benign reasons, e.g. because the underlying protocol does not
support shrinking of images or something.  Then we probably should carry on.


Yes, I agree here. If the bdrv_truncate() of bs->file failed, we can
print just a warning :) So, I'll send new version of the patch with
this change.


Max


Although if the shrink is almost complete, the truncation of bs->file
isn't so important thing and we could update qcow2 header.


I can live with the current version, though, so:

Reviewed-by: Max Reitz <address@hidden>

But I'll wait for a response from you before merging this series.

Max


+        if ((last_cluster + 1) * s->cluster_size < old_file_size) {
+            ret = bdrv_truncate(bs->file, (last_cluster + 1) *
s->cluster_size,
+                                PREALLOC_MODE_OFF, NULL);
+            if (ret < 0) {
+                error_setg_errno(errp, -ret,
+                                 "Failed to truncate the tail of the
image");
+                return ret;
+            }
+        }
       } else {
           ret = qcow2_grow_l1_table(bs, new_l1_size, true);
           if (ret < 0) {
diff --git a/block/qcow2.h b/block/qcow2.h
index 5a289a81e2..782a206ecb 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -597,6 +597,7 @@ int qcow2_change_refcount_order(BlockDriverState
*bs, int refcount_order,
                                   BlockDriverAmendStatusCB *status_cb,
                                   void *cb_opaque, Error **errp);
   int qcow2_shrink_reftable(BlockDriverState *bs);
+int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size);
     /* qcow2-cluster.c functions */
   int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
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