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Date: Fri, 16 Jan 2026 23:16:31 +0200
From: Leon Romanovsky <leon@kernel.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Chuck Lever <cel@kernel.org>, Jason Gunthorpe <jgg@nvidia.com>,
	linux-rdma@vger.kernel.org, linux-nfs@vger.kernel.org,
	NeilBrown <neilb@ownmail.net>, Jeff Layton <jlayton@kernel.org>,
	Olga Kornievskaia <okorniev@redhat.com>,
	Dai Ngo <dai.ngo@oracle.com>, Tom Talpey <tom@talpey.com>,
	Chuck Lever <chuck.lever@oracle.com>
Subject: Re: [PATCH v1 3/4] RDMA/core: add MR support for bvec-based RDMA
 operations
Message-ID: <20260116211631.GI14359@unreal>
References: <20260114143948.3946615-1-cel@kernel.org>
 <20260114143948.3946615-4-cel@kernel.org>
 <20260116114236.GG14359@unreal>
 <20260116145027.GA16842@lst.de>
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In-Reply-To: <20260116145027.GA16842@lst.de>

On Fri, Jan 16, 2026 at 03:50:27PM +0100, Christoph Hellwig wrote:
> On Fri, Jan 16, 2026 at 01:42:36PM +0200, Leon Romanovsky wrote:
> > On Wed, Jan 14, 2026 at 09:39:47AM -0500, Chuck Lever wrote:
> > > From: Chuck Lever <chuck.lever@oracle.com>
> > > 
> > > The bvec-based RDMA API currently returns -EOPNOTSUPP when Memory
> > > Region registration is required. This prevents iWARP devices from
> > > using the bvec path, since iWARP requires MR registration for RDMA
> > > READ operations. The force_mr debug parameter is also unusable with
> > > bvec input.
> > 
> > I am not very familiar with iWARP. Do you know why we need a special
> > case here? Is there a reason we cannot avoid using scatterlists for
> > iWARP as well, now or in the future?
> 
> iWarp must use MRs for the destination of RDMA READ operations, but the
> core RW code can also optionally use it for other things.  So to support
> that natively here we'd need a bvec-based version of ib_map_mr_sg.  Which
> would be really nice to have for the storage host drivers anyway, but
> until then the scatterlist emulation here can do.  And implementing it
> might take a while, as ib_map_mr_sg is a very thin wrapper around a call
> into the low-level driver.

It is in my roadmap, but as you said, it will take time :(.

Thanks