Re: [PATCH v4] migration/rdma: add x-rdma-chunk-size parameter

[Peter Xu <peterx@redhat.com>]

I'm looking at some RDMA security tickets recently, and I stumbled on top
of this chunk size problem.  Since this is very recently touched, I figured
maybe I should ask here,

On Mon, Mar 30, 2026 at 02:47:58PM +0800, Samuel Zhang wrote:
> @@ -2176,7 +2178,7 @@ static int qemu_rdma_write(RDMAContext *rdma,
>      rdma->current_length += len;
>  
>      /* flush it if buffer is too large */
> -    if (rdma->current_length >= RDMA_MERGE_MAX) {
> +    if (rdma->current_length >= rdma_merge_max()) {

[1]

What is this rdma_merge_max()?  Why it's twice the chunk size, and even if
I believe when reaching here the current_length should have been throttled
by chunk size too?

>          return qemu_rdma_write_flush(rdma, errp);
>      }

I mean, in qemu_rdma_buffer_mergeable():

    ...
    chunk_end = ram_chunk_end(block, rdma->current_chunk);
    ...
    if ((host_addr + len) > chunk_end) {
        return false;
    }

I believe it means when len will go beyond chunk size, it'll force a flush
already.

With that, when reaching [1] above, we either should have just flushed, or
we're under chunk size (which further guarantees smaller than
2*chunk_size).

What did I miss?  Or, is this line [1] dead code?

Thanks,

-- 
Peter Xu