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

[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). 
Could you detail how qcow2 does not have to be reentrant?
In below stack,
qcow2_update_snapshot_refcount
|- cpu_to_be64s(&l1_table[i])
|- bdrv_pwrite_sync
|-- bdrv_pwrite
|--- bdrv_pwritev
|---- bdrv_prwv_co
|----- aio_poll(aio_context) <== this aio_context is qemu_aio_context
|------ aio_dispatch
|------- bdrv_co_io_em_complete
|-------- qemu_coroutine_enter(co->coroutine, NULL); <== coroutine entry is 
bdrv_co_do_rw
bdrv_co_do_rw will access l1_table to perform I/O operation.

Thanks,
Zhang Haoyu
>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