Re: [Qemu-devel] [PATCH 3/8] migration: support to detectcompression and decompression errors

[jiang.biao2]

> On 03/27/2018 07:17 PM, Peter Xu wrote:
>> On Tue, Mar 27, 2018 at 03:42:32AM +0800, Xiao Guangrong wrote:
>> 
>> [...]
>> 
>>>> It'll be understandable to me if the problem is that the compress()
>>>> API does not allow the input buffer to be changed during the whole
>>>> period of the call.  If that is a must, this patch for sure helps.
>>>
>>> Yes, that is exactly what i want to say. :)
>> 
>> So I think now I know what this patch is for. :) And yeah, it makes
>> sense.
>> 
>> Though another question would be: if the buffer is updated during
>> compress() and compress() returned error, would that pollute the whole
>> z_stream or it only fails the compress() call?
>> 
>
> I guess deflateReset() can recover everything, i.e, keep z_stream as
> it is init'ed by deflate_init().
>
>> (Same question applies to decompress().)
>> 
>> If it's only a compress() error and it won't pollute z_stream (or say,
>> it can be recovered after a deflateReset() and then we can continue to
>> call deflate() without problem), then we'll actually have two
>> alternatives to solve this "buffer update" issue:
>> 
>> 1. Use the approach of current patch: we copy the page every time, so
>>     deflate() never fails because update never happens.  But it's slow
>>     since we copy the pages every time.
>> 
>> 2. Use the old approach, and when compress() fail, we just ignore that
>>     page (since now we know that error _must_ be caused by page update,
>>     then we are 100% sure that we'll send that page again so it'll be
>>     perfectly fine).
>> 
>
> No, we can't make the assumption that "error _must_ be caused by page 
> update". 
> No document/ABI about compress/decompress promised it. :)
So, as I metioned before, can we just distingush the decompress/compress errors 
from errors caused by page update by the return code of inflate/deflate?
According to the zlib manual, there seems to be several error codes for 
different 
cases,
#define Z_ERRNO        (-1) 
#define Z_STREAM_ERROR (-2) 
#define Z_DATA_ERROR   (-3) 
#define Z_MEM_ERROR    (-4)
#define Z_BUF_ERROR    (-5)
#define Z_VERSION_ERROR (-6)
Did you check the return code when silent failure(not caused by page update) 
happened before? :)

Jiang
Regards