netdev - Re: [PATCH net-next 2/3] mm: vmalloc: export find_vm

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Message-ID: <20260129113609.GA37734@j66a10360.sqa.eu95>
Date: Thu, 29 Jan 2026 19:36:09 +0800
From: "D. Wythe" <alibuda@...ux.alibaba.com    >
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: "D. Wythe" <alibuda@...ux.alibaba.com>,
	Leon Romanovsky <leon@...nel.org>,
	Uladzislau Rezki <urezki@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dust Li <dust.li@...ux.alibaba.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Sidraya Jayagond <sidraya@...ux.ibm.com>,
	Wenjia Zhang <wenjia@...ux.ibm.com>,
	Mahanta Jambigi <mjambigi@...ux.ibm.com>,
	Simon Horman <horms@...nel.org>, Tony Lu <tonylu@...ux.alibaba.com>,
	Wen Gu <guwen@...ux.alibaba.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-rdma@...r.kernel.org,
	linux-s390@...r.kernel.org, netdev@...r.kernel.org,
	oliver.yang@...ux.alibaba.com
Subject: Re: [PATCH net-next 2/3] mm: vmalloc: export find_vm_area()

On Wed, Jan 28, 2026 at 02:06:29PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 28, 2026 at 11:45:58AM +0800, D. Wythe wrote:
> 
> > By leveraging vmalloc_huge() and the proposed helper to increase the
> > page_size in ib_map_mr_sg(), each MTTE covers a much larger contiguous
> > physical block.
> 
> This doesn't seem right, if your goal is to take a vmalloc() pointer
> and convert it to a MR via a scatterlist and ib_map_mr_sg() then you
> should be asking for a helper to convert a kernel pointer into a
> scatterlist.
> 
> Even if you do this in a naive way and call the
> sg_alloc_append_table_from_pages() function it will automatically join
> physically contiguous ranges together for you.
> 
> From there you can check the resulting scatterlist and compute the
> page_size to pass to ib_map_mr_sg().
> 
> No need to ask the MM for anything other than the list of physicals to
> build the scatterlist with.
> 
> Still, I wouldn't mind seeing a helper to convert a kernel pointer
> into a scatterlist because I have see that opencoded in a few places,
> and maybe there are ways to optimize that using more information from
> the MM - but it should be APIs used only by this helper not exposed to
> drivers.
> 
> Jason

Hi Jason,

To be honest, I was previously unaware of the
sg_alloc_append_table_from_pages() function, although I had indeed
considered manually calculating the size of contiguous physical blocks.
The reason I proposed the MM helper is that SMC is not driver, it  utilizes
vmalloc() for memory allocation and is thus in direct contact with the MM;
from this perspective, having the MM provide the page_order would be the most
straightforward approach.

Given the significant opposition and our plans to transition SMC to newer
APIs in the future anyway, I agree that introducing this helper now is
less justified.

I will follow your suggestion and update the next version accordingly.

Thanks.