Re: [PATCH 1/3] multifd: bugfix for migration using compression methods

[Peter Xu]

On Wed, Dec 18, 2024 at 05:14:11PM +0800, Yuan Liu wrote:
> When compression is enabled on the migration channel and
> the pages processed are all zero pages, these pages will
> not be sent and updated on the target side, resulting in
> incorrect memory data on the source and target sides.
> 
> The root cause is that all compression methods call
> multifd_send_prepare_common to determine whether to compress
> dirty pages, but multifd_send_prepare_common does not update
> the IOV of MultiFDPacket_t when all dirty pages are zero pages.
> 
> The solution is to always update the IOV of MultiFDPacket_t
> regardless of whether the dirty pages are all zero pages.
> 
> Signed-off-by: Yuan Liu <yuan1.liu@intel.com>
> Reviewed-by: Jason Zeng <jason.zeng@intel.com>

Ouch.. thanks for digging this out.

Reviewed-by: Peter Xu <peterx@redhat.com>

Is this the correct Fixes tag (and copy stable for 9.0+)?

Fixes: 303e6f54f9 ("migration/multifd: Implement zero page transmission on the 
multifd thread.")

> ---
>  migration/multifd-nocomp.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c
> index 55191152f9..2e4aaac285 100644
> --- a/migration/multifd-nocomp.c
> +++ b/migration/multifd-nocomp.c
> @@ -362,6 +362,7 @@ int multifd_ram_flush_and_sync(void)
>  bool multifd_send_prepare_common(MultiFDSendParams *p)
>  {
>      MultiFDPages_t *pages = &p->data->u.ram;
> +    multifd_send_prepare_header(p);
>      multifd_send_zero_page_detect(p);
>  
>      if (!pages->normal_num) {
> @@ -369,8 +370,6 @@ bool multifd_send_prepare_common(MultiFDSendParams *p)
>          return false;
>      }
>  
> -    multifd_send_prepare_header(p);
> -
>      return true;
>  }
>  
> -- 
> 2.43.0
> 

-- 
Peter Xu