Re: [Qemu-devel] [PATCH] Fix block migration when the device size is not a multiple of 1 MB

[Kevin Wolf <kwolf@redhat.com>]

Am 21.01.2011 13:15, schrieb Yoshiaki Tamura:
> 2011/1/21 Pierre Riteau <Pierre.Riteau@irisa.fr>:
>> Le 20 janv. 2011 à 17:18, Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp> a écrit :
>>
>>> 2011/1/20 Pierre Riteau <Pierre.Riteau@irisa.fr>:
>>>> On 20 janv. 2011, at 03:06, Yoshiaki Tamura wrote:
>>>>
>>>>> 2011/1/19 Pierre Riteau <Pierre.Riteau@irisa.fr>:
>>>>>> b02bea3a85cc939f09aa674a3f1e4f36d418c007 added a check on the return
>>>>>> value of bdrv_write and aborts migration when it fails. However, if the
>>>>>> size of the block device to migrate is not a multiple of BLOCK_SIZE
>>>>>> (currently 1 MB), the last bdrv_write will fail with -EIO.
>>>>>>
>>>>>> Fixed by calling bdrv_write with the correct size of the last block.
>>>>>> ---
>>>>>>  block-migration.c |   16 +++++++++++++++-
>>>>>>  1 files changed, 15 insertions(+), 1 deletions(-)
>>>>>>
>>>>>> diff --git a/block-migration.c b/block-migration.c
>>>>>> index 1475325..eeb9c62 100644
>>>>>> --- a/block-migration.c
>>>>>> +++ b/block-migration.c
>>>>>> @@ -635,6 +635,8 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
>>>>>>     int64_t addr;
>>>>>>     BlockDriverState *bs;
>>>>>>     uint8_t *buf;
>>>>>> +    int64_t total_sectors;
>>>>>> +    int nr_sectors;
>>>>>>
>>>>>>     do {
>>>>>>         addr = qemu_get_be64(f);
>>>>>> @@ -656,10 +658,22 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
>>>>>>                 return -EINVAL;
>>>>>>             }
>>>>>>
>>>>>> +            total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
>>>>>> +            if (total_sectors <= 0) {
>>>>>> +                fprintf(stderr, "Error getting length of block device %s\n", device_name);
>>>>>> +                return -EINVAL;
>>>>>> +            }
>>>>>> +
>>>>>> +            if (total_sectors - addr < BDRV_SECTORS_PER_DIRTY_CHUNK) {
>>>>>> +                nr_sectors = total_sectors - addr;
>>>>>> +            } else {
>>>>>> +                nr_sectors = BDRV_SECTORS_PER_DIRTY_CHUNK;
>>>>>> +            }
>>>>>> +
>>>>>>             buf = qemu_malloc(BLOCK_SIZE);
>>>>>>
>>>>>>             qemu_get_buffer(f, buf, BLOCK_SIZE);
>>>>>> -            ret = bdrv_write(bs, addr, buf, BDRV_SECTORS_PER_DIRTY_CHUNK);
>>>>>> +            ret = bdrv_write(bs, addr, buf, nr_sectors);
>>>>>>
>>>>>>             qemu_free(buf);
>>>>>>             if (ret < 0) {
>>>>>> --
>>>>>> 1.7.3.5
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> Hi Pierre,
>>>>>
>>>>> I don't think the fix above is correct.  If you have a file which
>>>>> isn't aliened with BLOCK_SIZE, you won't get an error with the
>>>>> patch.  However, the receiver doesn't know how much sectors which
>>>>> the sender wants to be written, so the guest may fail after
>>>>> migration because some data may not be written.  IIUC, although
>>>>> changing bytestream should be prevented as much as possible, we
>>>>> should save/load total_sectors to check appropriate file is
>>>>> allocated on the receiver side.
>>>>
>>>> Isn't the guest supposed to be started using a file with the correct size?
>>>
>>> I personally don't like that; It's insisting too much to the user.
>>> Can't we expand the image on the fly?  We can just abort if expanding
>>> failed anyway.
>>
>> At first I thought your expansion idea was best, but now I think there are valid scenarios where it fails.
>>
>> Imagine both sides are not using a file but a disk partition as storage. If the partition size is not rounded to 1 MB, the last write will fail with the current code, and there is no way we can expand the partition.
>>
> 
> Right.  But in case of partition doesn't the check in the patch below
> return error?  Does bdrv_getlength return the size correctly?

I'm pretty sure that it does. We would have problems in other places if
it didn't (e.g. we're checking if I/O requests are within the disk size).

Kevin