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

[Yoshiaki Tamura]

2011/1/21 Kevin Wolf <address@hidden>:
> Am 21.01.2011 13:15, schrieb Yoshiaki Tamura:
>> 2011/1/21 Pierre Riteau <address@hidden>:
>>> Le 20 janv. 2011 à 17:18, Yoshiaki Tamura <address@hidden> a écrit :
>>>
>>>> 2011/1/20 Pierre Riteau <address@hidden>:
>>>>> On 20 janv. 2011, at 03:06, Yoshiaki Tamura wrote:
>>>>>
>>>>>> 2011/1/19 Pierre Riteau <address@hidden>:
>>>>>>> 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).

Sorry for the noise.  I just learned it's returning the value of lseek
in case of raw-posix.

Yoshi

>
> Kevin
>
>