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

[Pierre Riteau]

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?
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.

> 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/