Commit 00da250c authored Mar 13, 2026 by Chuck Lever Committed by Leon Romanovsky Mar 17, 2026

RDMA/rw: Fall back to direct SGE on MR pool exhaustion

When IOMMU passthrough mode is active, ib_dma_map_sgtable_attrs()
produces no coalescing: each scatterlist page maps 1:1 to a DMA
entry, so sgt.nents equals the raw page count. A 1 MB transfer
yields 256 DMA entries. If that count exceeds the device's
max_sgl_rd threshold (an optimization hint from mlx5 firmware),
rdma_rw_io_needs_mr() steers the operation into the MR
registration path. Each such operation consumes one or more MRs
from a pool sized at max_rdma_ctxs -- roughly one MR per
concurrent context. Under write-intensive workloads that issue
many concurrent RDMA READs, the pool is rapidly exhausted,
ib_mr_pool_get() returns NULL, and rdma_rw_init_one_mr() returns
-EAGAIN. Upper layer protocols treat this as a fatal DMA mapping
failure and tear down the connection.

The max_sgl_rd check is a performance optimization, not a
correctness requirement: the device can handle large SGE counts
via direct posting, just less efficiently than with MR
registration. When the MR pool cannot satisfy a request, falling
back to the direct SGE (map_wrs) path avoids the connection
reset while preserving the MR optimization for the common case
where pool resources are available.

Add a fallback in rdma_rw_ctx_init() so that -EAGAIN from
rdma_rw_init_mr_wrs() triggers direct SGE posting instead of
propagating the error. iWARP devices, which mandate MR
registration for RDMA READs, and force_mr debug mode continue
to treat -EAGAIN as terminal.

Fixes: 00bd1439 ("RDMA/rw: Support threshold for registration vs scattering to local pages")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260313194201.5818-2-cel@kernel.org

Signed-off-by: Leon Romanovsky <leon@kernel.org>

parent ef3b0674

drivers/infiniband/core/rw.c

+18 −3

Original line number	Diff line number	Diff line
		@@ -608,14 +608,29 @@ int rdma_rw_ctx_init(struct rdma_rw_ctx ctx, struct ib_qp qp, u32 port_num,
		if (rdma_rw_io_needs_mr(qp->device, port_num, dir, sg_cnt)) {
		ret = rdma_rw_init_mr_wrs(ctx, qp, port_num, sg, sg_cnt,
		sg_offset, remote_addr, rkey, dir);
		} else if (sg_cnt > 1) {
		/*
		* If MR init succeeded or failed for a reason other
		* than pool exhaustion, that result is final.
		*
		* Pool exhaustion (-EAGAIN) from the max_sgl_rd
		* optimization is recoverable: fall back to
		* direct SGE posting. iWARP and force_mr require
		* MRs unconditionally, so -EAGAIN is terminal.
		*/
		if (ret != -EAGAIN \|\|
		rdma_protocol_iwarp(qp->device, port_num) \|\|
		unlikely(rdma_rw_force_mr))
		goto out;
		}

		if (sg_cnt > 1)
		ret = rdma_rw_init_map_wrs(ctx, qp, sg, sg_cnt, sg_offset,
		remote_addr, rkey, dir);
		} else {
		else
		ret = rdma_rw_init_single_wr(ctx, qp, sg, sg_offset,
		remote_addr, rkey, dir);
		}

		out:
		if (ret < 0)
		goto out_unmap_sg;
		return ret;