Re: [PATCH v4 8/9] migration/ram: Factor out populating pages readable in ram_block_populate_pages()

[Peter Xu <peterx@redhat.com>]

On Thu, Sep 02, 2021 at 03:14:31PM +0200, David Hildenbrand wrote:
> Let's factor out prefaulting/populating to make further changes easier to
> review. While at it, use the actual page size of the ramblock, which
> defaults to qemu_real_host_page_size for anonymous memory.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  migration/ram.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index e1c158dc92..de47650c90 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1639,6 +1639,17 @@ out:
>      return ret;
>  }
>  
> +static inline void populate_range(RAMBlock *block, ram_addr_t offset,
> +                                  ram_addr_t size)
> +{
> +    for (; offset < size; offset += block->page_size) {
> +        char tmp = *((char *)block->host + offset);
> +
> +        /* Don't optimize the read out */
> +        asm volatile("" : "+r" (tmp));
> +    }
> +}

If to make it a common function, make it populate_range_read()?

Just to identify from RW, as we'll fill the holes with zero pages only, not
doing page allocations yet, so not a complete "populate".

That'll be good enough for live snapshot as uffd-wp works for zero pages,
however I'm just afraid it may stop working for some new users of it when zero
pages won't suffice.

Maybe some comment would help too?

-- 
Peter Xu