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

[address@hidden]

On 22/08/2021 16:53, Marcel Apfelbaum wrote:
> 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)...

Good idea !



>
> Also, I  am not (personally) too fond of the "on_demand" label usage here,
> however I will let the maintainer/others decide.
Indeed, how just repeating the ibv_reg_mr() instead of a 'go to'

Thanks
Zhijian



>
> 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
>>
>>
>>
>>
>