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

[Yoshiaki Tamura]

2011/1/21 Pierre Riteau <address@hidden>:
> On 21 janv. 2011, at 13:36, Yoshiaki Tamura wrote:
>
>> 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.
>
>
> And it does a ioctl call on other platforms than Linux.

Thanks.  Just a quick question regarding total_sectors.
BlockDriverState seems to contain total_sectors.  Can we avoid
calling bdrv_getlength() if bs->total_sectors were already there?

Yoshi

>
> --
> Pierre Riteau -- PhD student, Myriads team, IRISA, Rennes, France
> http://perso.univ-rennes1.fr/pierre.riteau/
>
>
>