Re: [Qemu-devel] NVDIMM live migration broken?

[Juan Quintela <quintela@redhat.com>]

Haozhong Zhang <haozhong.zhang@intel.com> wrote:

....

Hi

I am trying to see what is going on.

>> 
>
> I managed to reproduce this bug. After bisect between good v2.8.0 and
> bad edf8bc984, it looks a regression introduced by
>     6b6712efccd "ram: Split dirty bitmap by RAMBlock"
> This commit may result in guest crash after migration if any host
> memory backend is used.
>
> Could you test whether the attached draft patch fixes this bug? If yes,
> I will make a formal patch later.
>
> Thanks,
> Haozhong
>
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index 73d1bea8b6..2ae4ff3965 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -377,7 +377,9 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>                                                 uint64_t *real_dirty_pages)
>  {
>      ram_addr_t addr;
> +    ram_addr_t offset = rb->offset;
>      unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
> +    unsigned long dirty_page = BIT_WORD((start + offset) >> TARGET_PAGE_BITS);
>      uint64_t num_dirty = 0;
>      unsigned long *dest = rb->bmap;
>  


If this is the case, I can't understand how it ever worked :-(

Investigating.

Later, Juan.

> @@ -386,8 +388,9 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>          int k;
>          int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
>          unsigned long * const *src;
> -        unsigned long idx = (page * BITS_PER_LONG) / DIRTY_MEMORY_BLOCK_SIZE;
> -        unsigned long offset = BIT_WORD((page * BITS_PER_LONG) %
> +        unsigned long idx = (dirty_page * BITS_PER_LONG) /
> +                            DIRTY_MEMORY_BLOCK_SIZE;
> +        unsigned long offset = BIT_WORD((dirty_page * BITS_PER_LONG) %
>                                          DIRTY_MEMORY_BLOCK_SIZE);
>  
>          rcu_read_lock();
> @@ -416,7 +419,7 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>      } else {
>          for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
>              if (cpu_physical_memory_test_and_clear_dirty(
> -                        start + addr,
> +                        start + addr + offset,
>                          TARGET_PAGE_SIZE,
>                          DIRTY_MEMORY_MIGRATION)) {
>                  *real_dirty_pages += 1;