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

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Message-ID: <20260106010709.GO125261@ziepe.ca>
Date: Mon, 5 Jan 2026 21:07:09 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
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 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.

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..

Jason