Re: [PATCH for-next v2 2/2] RDMA/rxe: Enable asynchronous prefetch for ODP MRs

[Daisuke Matsuda <dskmtsd@gmail.com>]

On 2025/05/06 0:25, Zhu Yanjun wrote:
> On 03.05.25 15:42, Daisuke Matsuda wrote:
>> Calling ibv_advise_mr(3) with flags other than IBV_ADVISE_MR_FLAG_FLUSH
>> invokes asynchronous requests. It is best-effort, and thus can safely be
>> deferred to the system-wide workqueue.
>>
>> Signed-off-by: Daisuke Matsuda <dskmtsd@gmail.com>
> 
> I have made tests with rdma-core after applying this patch series. It seems that it can work well.
> I read through this commit. Other than the following minor problems, I am fine with this commit.
> 
> Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
> 
>> ---
>>   drivers/infiniband/sw/rxe/rxe_odp.c | 81 ++++++++++++++++++++++++++++-
>>   1 file changed, 80 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/infiniband/sw/rxe/rxe_odp.c b/drivers/infiniband/sw/rxe/rxe_odp.c
>> index e5c60b061d7e..d98b385a18ce 100644
>> --- a/drivers/infiniband/sw/rxe/rxe_odp.c
>> +++ b/drivers/infiniband/sw/rxe/rxe_odp.c
>> @@ -425,6 +425,73 @@ enum resp_states rxe_odp_do_atomic_write(struct rxe_mr *mr, u64 iova, u64 value)
>>       return RESPST_NONE;
>>   }
>> +struct prefetch_mr_work {
>> +    struct work_struct work;
>> +    u32 pf_flags;
>> +    u32 num_sge;
>> +    struct {
>> +        u64 io_virt;
>> +        struct rxe_mr *mr;
>> +        size_t length;
>> +    } frags[];
>> +};
> 
> The struct prefetch_mr_work should be moved into header file? IMO, it is better to move this struct to rxe_loc.h?

This struct is not likely to be used in other files.
I think leaving it here would be easier for other developers to understand because relevant codes are gathered.
If there is any specific reason to move, I will do so.

> 
>> +
>> +static void rxe_ib_prefetch_mr_work(struct work_struct *w)
>> +{
>> +    struct prefetch_mr_work *work =
>> +        container_of(w, struct prefetch_mr_work, work);
>> +    int ret;
>> +    u32 i;
>> +
>> +    /* We rely on IB/core that work is executed if we have num_sge != 0 only. */
>> +    WARN_ON(!work->num_sge);
>> +    for (i = 0; i < work->num_sge; ++i) {
>> +        struct ib_umem_odp *umem_odp;
>> +
>> +        ret = rxe_odp_do_pagefault_and_lock(work->frags[i].mr, work->frags[i].io_virt,
>> +                            work->frags[i].length, work->pf_flags);
>> +        if (ret < 0) {
>> +            rxe_dbg_mr(work->frags[i].mr, "failed to prefetch the mr\n");
>> +            continue;
>> +        }
>> +
>> +        umem_odp = to_ib_umem_odp(work->frags[i].mr->umem);
>> +        mutex_unlock(&umem_odp->umem_mutex);
> 
> Obviously this function is dependent on the mutex lock umem_mutex. So in the beginning of this function, it is better to  add lockdep_assert_held(&umem_odp->umem_mutex)?

The mutex **must not** be locked at the beginning, so
perhaps we can add lockdep_assert_not_held() instead at the beginning,
but this one is not used frequently, and we can do without that.

umem_mutex is locked in rxe_odp_do_pagefault_and_lock() in the for loop.
The function calls ib_umem_odp_map_dma_and_lock(), which locks the mutex only when pagefault is successful.
If ib_umem_odp_map_dma_and_lock() fails, an error is returned and then the mutex is not locked.

Daisuke

> 
> Zhu Yanjun
> 
>> +    }
>> +
>> +    kvfree(work);
>> +}
>> +
>> +static int rxe_init_prefetch_work(struct ib_pd *ibpd,
>> +                  enum ib_uverbs_advise_mr_advice advice,
>> +                  u32 pf_flags, struct prefetch_mr_work *work,
>> +                  struct ib_sge *sg_list, u32 num_sge)
>> +{
>> +    struct rxe_pd *pd = container_of(ibpd, struct rxe_pd, ibpd);
>> +    u32 i;
>> +
>> +    INIT_WORK(&work->work, rxe_ib_prefetch_mr_work);
>> +    work->pf_flags = pf_flags;
>> +
>> +    for (i = 0; i < num_sge; ++i) {
>> +        struct rxe_mr *mr;
>> +
>> +        mr = lookup_mr(pd, IB_ACCESS_LOCAL_WRITE,
>> +                   sg_list[i].lkey, RXE_LOOKUP_LOCAL);
>> +        if (IS_ERR(mr)) {
>> +            work->num_sge = i;
>> +            return PTR_ERR(mr);
>> +        }
>> +        work->frags[i].io_virt = sg_list[i].addr;
>> +        work->frags[i].length = sg_list[i].length;
>> +        work->frags[i].mr = mr;
>> +
>> +        rxe_put(mr);
>> +    }
>> +    work->num_sge = num_sge;
>> +    return 0;
>> +}
>> +
>>   static int rxe_ib_prefetch_sg_list(struct ib_pd *ibpd,
>>                      enum ib_uverbs_advise_mr_advice advice,
>>                      u32 pf_flags, struct ib_sge *sg_list,
>> @@ -478,6 +545,8 @@ static int rxe_ib_advise_mr_prefetch(struct ib_pd *ibpd,
>>                        u32 flags, struct ib_sge *sg_list, u32 num_sge)
>>   {
>>       u32 pf_flags = RXE_PAGEFAULT_DEFAULT;
>> +    struct prefetch_mr_work *work;
>> +    int rc;
>>       if (advice == IB_UVERBS_ADVISE_MR_ADVICE_PREFETCH)
>>           pf_flags |= RXE_PAGEFAULT_RDONLY;
>> @@ -490,7 +559,17 @@ static int rxe_ib_advise_mr_prefetch(struct ib_pd *ibpd,
>>           return rxe_ib_prefetch_sg_list(ibpd, advice, pf_flags, sg_list,
>>                              num_sge);
>> -    /* Asynchronous call is "best-effort" */
>> +    /* Asynchronous call is "best-effort" and allowed to fail */
>> +    work = kvzalloc(struct_size(work, frags, num_sge), GFP_KERNEL);
>> +    if (!work)
>> +        return -ENOMEM;
>> +
>> +    rc = rxe_init_prefetch_work(ibpd, advice, pf_flags, work, sg_list, num_sge);
>> +    if (rc) {
>> +        kvfree(work);
>> +        return rc;
>> +    }
>> +    queue_work(system_unbound_wq, &work->work);
>>       return 0;
>>   }
>