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

[Xiao Guangrong]

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. :)

Thanks!