Re: [Qemu-devel] [question] is it possible that big-endian l1 tableoffset referenced by other I/O while updating l1 table offset in qcow2_update_snapshot_refcount?

[Max Reitz <mreitz@redhat.com>]

Am 13.10.2014 um 05:17 schrieb Zhang Haoyu:
>>>>> Hi,
>>>>> I encounter a problem that after deleting snapshot, the qcow2 image size is very larger than that it should be displayed by ls command,
>>>>> but the virtual disk size is okay via qemu-img info.
>>>>> I suspect that during updating l1 table offset, other I/O job reference the big-endian l1 table offset (very large value),
>>>>> so the file is truncated to very large.
>>>> Not quite.  Rather, all the data that the snapshot used to occupy is
>>>> still consuming holes in the file; the maximum offset of the file is
>>>> still unchanged, even if the file is no longer using as many referenced
>>>> clusters.  Recent changes have gone in to sparsify the file when
>>>> possible (punching holes if your kernel and file system is new enough to
>>>> support that), so that it is not consuming the amount of disk space that
>>>> a mere ls reports.  But if what you are asking for is a way to compact
>>>> the file back down, then you'll need to submit a patch.  The idea of
>>>> having an online defragmenter for qcow2 files has been kicked around
>>>> before, but it is complex enough that no one has attempted a patch yet.
>>> Sorry, I didn't clarify the problem clearly.
>>> In qcow2_update_snapshot_refcount(), below code,
>>>       /* Update L1 only if it isn't deleted anyway (addend = -1) */
>>>       if (ret == 0 && addend >= 0 && l1_modified) {
>>>           for (i = 0; i < l1_size; i++) {
>>>               cpu_to_be64s(&l1_table[i]);
>>>           }
>>>
>>>           ret = bdrv_pwrite_sync(bs->file, l1_table_offset, l1_table, l1_size2);
>>>
>>>           for (i = 0; i < l1_size; i++) {
>>>               be64_to_cpus(&l1_table[i]);
>>>           }
>>>       }
>>> between cpu_to_be64s(&l1_table[i]); and be64_to_cpus(&l1_table[i]);,
>>> is it possible that there is other I/O reference this interim l1 table whose entries contain the be64 l2 table offset?
>>> The be64 l2 table offset maybe a very large value, hundreds of TB is possible,
>>> then the qcow2 file will be truncated to far larger than normal size.
>>> So we'll see the huge size of the qcow2 file by ls -hl, but the size is still normal displayed by qemu-img info.
>>>
>>> If the possibility mentioned above exists, below raw code may fix it,
>>>        if (ret == 0 && addend >= 0 && l1_modified) {
>>>           tmp_l1_table = g_malloc0(l1_size * sizeof(uint64_t))
>>>           memcpy(tmp_l1_table, l1_table, l1_size * sizeof(uint64_t));
>>>           for (i = 0; i < l1_size; i++) {
>>>               cpu_to_be64s(&tmp_l1_table[i]);
>>>           }
>>>           ret = bdrv_pwrite_sync(bs->file, l1_table_offset, tmp_l1_table, l1_size2);
>>>
>>>           free(tmp_l1_table);
>>>       }
>> l1_table is already a local variable (local to
>> qcow2_update_snapshot_refcount()), so I can't really imagine how
>> introducing another local buffer should mitigate the problem, if there
>> is any.
>>
> l1_table is not necessarily a local variable to qcow2_update_snapshot_refcount,
> which depends on condition of "if (l1_table_offset != s->l1_table_offset)",
> if the condition not true, l1_table = s->l1_table.

Oh, yes, you're right. Okay, so in theory nothing should happen anyway, 
because qcow2 does not have to be reentrant (so s->l1_table will not be 
accessed while it's big endian and therefore possibly not in CPU order). 
But I find it rather ugly to convert the cached L1 table to big endian, 
so I'd be fine with the patch you proposed.

Max