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

[Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>]

2011/1/19 Pierre Riteau <Pierre.Riteau@irisa.fr>:
> 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.

BTW, you should use error_report instead of fprintf(stderr, ...).

Thanks,

Yoshi