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Message-ID: <2a6081b8-1772-4064-97d8-70d636b1868e@gmail.com>
Date: Fri, 9 May 2025 21:19:08 +0900
From: Daisuke Matsuda <dskmtsd@...il.com>
To: Zhu Yanjun <yanjun.zhu@...ux.dev>, linux-kernel@...r.kernel.org,
linux-rdma@...r.kernel.org, leon@...nel.org, jgg@...pe.ca,
zyjzyj2000@...il.com
Subject: Re: [PATCH for-next v2 2/2] RDMA/rxe: Enable asynchronous prefetch
for ODP MRs
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@...il.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@...ux.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;
>> }
>
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