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

[Yoshiaki Tamura]

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.

> But I guess changing the protocol would be best as it would avoid headaches 
> to people who mistakenly created a file that is too small.

We should think carefully before changing the protocol.

Kevin?

>
>> BTW, you should use error_report instead of fprintf(stderr, ...).
>
> I didn't know that, I followed what was used in this file. Thank you.
>
> --
> Pierre Riteau -- PhD student, Myriads team, IRISA, Rennes, France
> http://perso.univ-rennes1.fr/pierre.riteau/
>
>
>