Re: [PATCH v2] RDMA/mlx5: Wait for in-flight page faults on implicit MR null_mkey dereg

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] RDMA/mlx5: child MR leak in mlx5_ib_free_odp_mr due to late null_mmkey wait
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commit f5fc9e80765babb54d4d8c2db70efadd75e2ccc8
Author: Li RongQing <lirongqing@baidu.com>

    RDMA/mlx5: Wait for in-flight page faults on implicit MR null_mkey dereg

This commit mirrors the main-mmkey handling for null_mmkey by dropping the
reference and waiting for any in-flight faults to finish before destroying
the mkey and freeing the imr. It also initializes the waitqueue for the
null_mmkey right after creation to avoid a NULL pointer dereference.

> diff --git a/drivers/infiniband/hw/mlx5/odp.c b/drivers/infiniband/hw/mlx5/odp.c
> index 8dcbb0e5ccd7c..c7367718e24f2 100644
> --- a/drivers/infiniband/hw/mlx5/odp.c
> +++ b/drivers/infiniband/hw/mlx5/odp.c
[ ... ]
> @@ -683,6 +684,8 @@ void mlx5_ib_free_odp_mr(struct mlx5_ib_mr *mr)
>  		xa_erase(&mr_to_mdev(mr)->odp_mkeys,
>  			 mlx5_base_mkey(mr->null_mmkey.key));
>  
> +		mlx5r_deref_wait_odp_mkey(&mr->null_mmkey);
> +

[Severity: High]
Does this code leak child MRs when an in-flight page fault races with MR
deregistration?

If a page fault on an implicit ODP MR uses the null_mmkey and is queued
concurrently with MR deregistration, __mlx5_ib_dereg_mr() will call
mlx5_ib_free_odp_mr() which iterates over and deletes existing child MRs.

Because the null_mmkey is erased and waited upon here, after the
mr->implicit_children xarray is already cleaned up, the in-flight page
fault handler can still look up the null_mmkey, take a reference, and
create a new child MR.

The new child is inserted into mr->implicit_children (which succeeds
because the slot was just emptied by the cleanup loop). Finally,
mlx5_ib_free_odp_mr() waits for the handlers to finish. The parent mr is
freed, but the newly created child MR is permanently leaked and retains a
dangling parent pointer.

>  		mlx5_core_destroy_mkey(mr_to_mdev(mr)->mdev,
>  				       mr->null_mmkey.key);
>  	}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260924065017.2424-1-lirongqing@baidu.com?part=1