Re: [PATCH 1/2] migration/rdma: Try to register On-Demand Paging memory region

[Marcel Apfelbaum]

Hi

On Sat, Jul 31, 2021 at 5:00 PM Li Zhijian <lizhijian@cn.fujitsu.com> wrote:
>
> Previously, for the fsdax mem-backend-file, it will register failed with
> Operation not supported. In this case, we can try to register it with
> On-Demand Paging[1] like what rpma_mr_reg() does on rpma[2].
>
> [1]: 
> https://community.mellanox.com/s/article/understanding-on-demand-paging--odp-x
> [2]: http://pmem.io/rpma/manpages/v0.9.0/rpma_mr_reg.3
> Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
> ---
>  migration/rdma.c       | 27 ++++++++++++++++++---------
>  migration/trace-events |  1 +
>  2 files changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/migration/rdma.c b/migration/rdma.c
> index 5c2d113aa94..8784b5f22a6 100644
> --- a/migration/rdma.c
> +++ b/migration/rdma.c
> @@ -1123,15 +1123,21 @@ static int qemu_rdma_reg_whole_ram_blocks(RDMAContext 
> *rdma)
>      RDMALocalBlocks *local = &rdma->local_ram_blocks;
>
>      for (i = 0; i < local->nb_blocks; i++) {
> +        int access = IBV_ACCESS_LOCAL_WRITE | IBV_ACCESS_REMOTE_WRITE;
> +
> +on_demand:
>          local->block[i].mr =
>              ibv_reg_mr(rdma->pd,
>                      local->block[i].local_host_addr,
> -                    local->block[i].length,
> -                    IBV_ACCESS_LOCAL_WRITE |
> -                    IBV_ACCESS_REMOTE_WRITE
> +                    local->block[i].length, access
>                      );
>          if (!local->block[i].mr) {
> -            perror("Failed to register local dest ram block!");
> +            if (!(access & IBV_ACCESS_ON_DEMAND) && errno == ENOTSUP) {
> +                access |= IBV_ACCESS_ON_DEMAND;
> +                trace_qemu_rdma_register_odp_mr(local->block[i].block_name);
> +                goto on_demand;

Wouldn't it be better to check first if the device supports ODP ?
Something like:
    ret = ibv_exp_query_device(context, &dattr);
    if (dattr.exp_device_cap_flags & IBV_EXP_DEVICE_ODP)...

Also, I  am not (personally) too fond of the "on_demand" label usage here,
however I will let the maintainer/others decide.

Thanks,
Marcel

> +            }
> +            perror("Failed to register local dest ram block!\n");
>              break;
>          }
>          rdma->total_registrations++;
> @@ -1215,15 +1221,18 @@ static int 
> qemu_rdma_register_and_get_keys(RDMAContext *rdma,
>       */
>      if (!block->pmr[chunk]) {
>          uint64_t len = chunk_end - chunk_start;
> +        int access = rkey ? IBV_ACCESS_LOCAL_WRITE | IBV_ACCESS_REMOTE_WRITE 
> : 0;
>
>          trace_qemu_rdma_register_and_get_keys(len, chunk_start);
>
> -        block->pmr[chunk] = ibv_reg_mr(rdma->pd,
> -                chunk_start, len,
> -                (rkey ? (IBV_ACCESS_LOCAL_WRITE |
> -                        IBV_ACCESS_REMOTE_WRITE) : 0));
> -
> +on_demand:
> +        block->pmr[chunk] = ibv_reg_mr(rdma->pd, chunk_start, len, access);
>          if (!block->pmr[chunk]) {
> +            if (!(access & IBV_ACCESS_ON_DEMAND) && errno == ENOTSUP) {
> +                access |= IBV_ACCESS_ON_DEMAND;
> +                trace_qemu_rdma_register_odp_mr(block->block_name);
> +                goto on_demand;
> +            }
>              perror("Failed to register chunk!");
>              fprintf(stderr, "Chunk details: block: %d chunk index %d"
>                              " start %" PRIuPTR " end %" PRIuPTR
> diff --git a/migration/trace-events b/migration/trace-events
> index a1c0f034ab8..5f6aa580def 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -212,6 +212,7 @@ qemu_rdma_poll_write(const char *compstr, int64_t comp, 
> int left, uint64_t block
>  qemu_rdma_poll_other(const char *compstr, int64_t comp, int left) "other 
> completion %s (%" PRId64 ") received left %d"
>  qemu_rdma_post_send_control(const char *desc) "CONTROL: sending %s.."
>  qemu_rdma_register_and_get_keys(uint64_t len, void *start) "Registering %" 
> PRIu64 " bytes @ %p"
> +qemu_rdma_register_odp_mr(const char *name) "Try to register On-Demand 
> Paging memory region: %s"
>  qemu_rdma_registration_handle_compress(int64_t length, int index, int64_t 
> offset) "Zapping zero chunk: %" PRId64 " bytes, index %d, offset %" PRId64
>  qemu_rdma_registration_handle_finished(void) ""
>  qemu_rdma_registration_handle_ram_blocks(void) ""
> --
> 2.31.1
>
>
>
>