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

[Pierre Riteau]

On 21 janv. 2011, at 15:21, Yoshiaki Tamura wrote:

> 2011/1/21 Pierre Riteau <address@hidden>:
>> On 21 janv. 2011, at 14:59, Yoshiaki Tamura wrote:
>> 
>>> 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?
>> 
>> From a comment in bdrv_getlength():
>> 
>> Fixed size devices use the total_sectors value for speed instead of
>> issuing a length query (like lseek) on each call.  Also, legacy block
>> drivers don't provide a bdrv_getlength function and must use
>> total_sectors.
>> 
>> So using bdrv_getlength will protect against devices being resized during 
>> migration, but as far as I can see, the sender side doesn't support it: the 
>> value of total_sectors is cached for the whole block migration.
> 
> Even if the sender supports it, as far as total_sectors isn't
> sent to the receiver, can we follow the resize on the receiver?


I was referring to the complex, and probably unrealistic scenario, where a user 
allocates a file of the correct size on the receiving side, starts block 
migration, and during migration grows the size of the disk on both the sender 
and receiver side.

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