linux-kernel - Re: [PATCH RFC] rxe: Fix iova-to-va conversion for MR page sizes != PAGE

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Message-ID: <38f06731-1547-4057-89e4-5d0d5d5b2508@fujitsu.com>
Date: Wed, 7 Jan 2026 03:02:27 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: Jason Gunthorpe <jgg@...pe.ca>
CC: "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"zyjzyj2000@...il.com" <zyjzyj2000@...il.com>, "leon@...nel.org"
	<leon@...nel.org>, Yi Zhang <yi.zhang@...hat.com>, Bob Pearson
	<rpearsonhpe@...il.com>
Subject: Re: [PATCH RFC] rxe: Fix iova-to-va conversion for MR page sizes !=
 PAGE_SIZE



On 06/01/2026 09:07, Jason Gunthorpe wrote:
> On Mon, Jan 05, 2026 at 06:55:22AM +0000, Zhijian Li (Fujitsu) wrote:
> 
>> After digging into the behavior during the srp/012 test again, it
>> turns out this fix is incomplete.  The current xarray page_list
>> approach cannot correctly map memory regions composed of two or more
>> scatter-gather segments.
> I seem to recall there are DMA API functions that can control what
> kinds of scatterlists the block stack will push down.
> 
> For real HW we already cannot support less than 4K alignment of interior SGL
> segments.
> 
> Maybe rxe can tell the block stack it can only support PAGE_SIZE
> alignment of interior SGL segments?
> 
> If not then this would be the reason rxe needs mr->page_size, to
> support 4k.
> 

I agree that we should support smaller page sizes like 4K.
Some ULPs indeed have hardcoded assumptions about it.


> And obviously if the mr->page size is less than PAGE_SIZE the xarray
> datastructure does not work. You'd have to store physical addresses
> instead..



You're absolutely right that the current xarray of struct page pointers
is fundamentally flawed for this use case (both for mr->page_size < PAGE_SIZE
and for non-PAGE_SIZE aligned interior segments).

Storing the DMA addresses directly, as you suggested, seems like a much
more robust path forward. I will explore this approach.
This seems to revert back to 592627ccbdff ("RDMA/rxe: Replace rxe_map and rxe_phys_buf by xarray")

Thanks
Zhijian

> 
> Jason