linux-kernel - Re: [PATCH] mm/hmm: Allow hmm_dma_map

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Message-ID: <56b45ad5-b3ae-40f2-bf3f-d0718e3d26c7@gmail.com>
Date: Sat, 24 May 2025 00:56:07 +0900
From: Daisuke Matsuda <dskmtsd@...il.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: linux-rdma@...r.kernel.org, linux-mm@...ck.org, leon@...nel.org,
 jgg@...pe.ca, akpm@...ux-foundation.org, jglisse@...hat.com,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, zyjzyj2000@...il.com
Subject: Re: [PATCH] mm/hmm: Allow hmm_dma_map_alloc() to tolerate NULL device


On 2025/05/24 0:42, Christoph Hellwig wrote:
> Thank you very much, but I know rxe very well.  And given your apparent
> knowledge of the rdma subsystem you should also know pretty well that
> it does not otherwise call into the dma mapping core for virtual devices
> because calling into the dma mapping code is not valid for the virtual
> devices.
> 
> Please fix the rdma core to not call into the hmm dma mapping helpers
> for the ib_uses_virt_dma() case.
> 

Thank you for the clarification and guidance.

I'll look into updating the RDMA core to avoid calling hmm_dma_map_alloc() when ib_uses_virt_dma() is true. That should help keep the layering and responsibilities properly separated.

Thanks again,
Daisuke