Re: [Qemu-devel] [PATCH v2 3/3] dmg: don't skip zero chunk

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sun, Dec 23, 2018 at 10:59:39AM +0800, yuchenlin wrote:
> The dmg file has many tables which describe: "start from sector XXX to
> sector XXX, the compression method is XXX and where the compressed data
> resides on".
> 
> Each sector in the expanded file should be covered by a table. The table
> will describe the offset of compressed data (or raw depends on the type)
> in the dmg.
> 
> For example:
> 
> [-----------The expanded file------------]
> [---bzip table ---]/* zeros */[---zlib---]
>     ^
>     | if we want to read this sector.
> 
> we will find bzip table which contains this sector, and get the
> compressed data offset, read it from dmg, uncompress it, finally write to
> expanded file.
> 
> If we skip zero chunk (table), some sector cannot find the table which
> will cause search_chunk() return s->n_chunks, dmg_read_chunk() return -1
> and finally causing dmg_co_preadv() return EIO.
> 
> See:
> 
> [-----------The expanded file------------]
> [---bzip table ---]/* zeros */[---zlib---]
>                     ^
>                     | if we want to read this sector.
> 
> Oops, we cannot find the table contains it...
> 
> In the original implementation, we don't have zero table. When we try to
> read sector inside the zero chunk. We will get EIO, and skip reading.
> 
> After this patch, we treat zero chunk the same as ignore chunk, it will
> directly write zero and avoid some sector may not find the table.
> 
> After this patch:
> 
> [-----------The expanded file------------]
> [---bzip table ---][--zeros--][---zlib---]
> 
> Signed-off-by: yuchenlin <npes87184@gmail.com>
> ---
>  block/dmg.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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