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

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Message-ID: <20260128034558.GA126415@j66a10360.sqa.eu95>
Date: Wed, 28 Jan 2026 11:45:58 +0800
From: "D. Wythe" <alibuda@...ux.alibaba.com    >
To: Leon Romanovsky <leon@...nel.org>
Cc: "D. Wythe" <alibuda@...ux.alibaba.com>,
	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 Tue, Jan 27, 2026 at 03:34:17PM +0200, Leon Romanovsky wrote:
> On Sat, Jan 24, 2026 at 10:57:54PM +0800, D. Wythe wrote:
> > On Sat, Jan 24, 2026 at 11:48:59AM +0100, Uladzislau Rezki wrote:
> > > Hello, D. Wythe!
> > > 
> > > > On Fri, Jan 23, 2026 at 07:55:17PM +0100, Uladzislau Rezki wrote:
> > > > > On Fri, Jan 23, 2026 at 04:23:48PM +0800, D. Wythe wrote:
> > > > > > find_vm_area() provides a way to find the vm_struct associated with a
> > > > > > virtual address. Export this symbol to modules so that modularized
> > > > > > subsystems can perform lookups on vmalloc addresses.
> > > > > > 
> > > > > > Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>
> > > > > > ---
> > > > > >  mm/vmalloc.c | 1 +
> > > > > >  1 file changed, 1 insertion(+)
> > > > > > 
> > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > > index ecbac900c35f..3eb9fe761c34 100644
> > > > > > --- a/mm/vmalloc.c
> > > > > > +++ b/mm/vmalloc.c
> > > > > > @@ -3292,6 +3292,7 @@ struct vm_struct *find_vm_area(const void *addr)
> > > > > >  
> > > > > >  	return va->vm;
> > > > > >  }
> > > > > > +EXPORT_SYMBOL_GPL(find_vm_area);
> > > > > >  
> > > > > This is internal. We can not just export it.
> > > > > 
> > > > > --
> > > > > Uladzislau Rezki
> > > > 
> > > > Hi Uladzislau,
> > > > 
> > > > Thank you for the feedback. I agree that we should avoid exposing
> > > > internal implementation details like struct vm_struct to external
> > > > subsystems.
> > > > 
> > > > Following Christoph's suggestion, I'm planning to encapsulate the page
> > > > order lookup into a minimal helper instead:
> > > > 
> > > > unsigned int vmalloc_page_order(const void *addr){
> > > > 	struct vm_struct *vm;
> > > >  	vm = find_vm_area(addr);
> > > > 	return vm ? vm->page_order : 0;
> > > > }
> > > > EXPORT_SYMBOL_GPL(vmalloc_page_order);
> > > > 
> > > > Does this approach look reasonable to you? It would keep the vm_struct
> > > > layout private while satisfying the optimization needs of SMC.
> > > > 
> > > Could you please clarify why you need info about page_order? I have not
> > > looked at your second patch.
> > > 
> > > Thanks!
> > > 
> > > --
> > > Uladzislau Rezki
> > 
> > Hi Uladzislau,
> > 
> > This stems from optimizing memory registration in SMC-R. To provide the
> > RDMA hardware with direct access to memory buffers, we must register
> > them with the NIC. During this process, the hardware generates one MTT
> > entry for each physically contiguous block. Since these hardware entries
> > are a finite and scarce resource, and SMC currently defaults to a 4KB
> > registration granularity, a single 2MB buffer consumes 512 entries. In
> > high-concurrency scenarios, this inefficiency quickly exhausts NIC
> > resources and becomes a major bottleneck for system scalability.
> 
> I believe this complexity can be avoided by using the RDMA MR pool API,
> as other ULPs do, for example NVMe.
> 
> Thanks
> 

Hi Leon,

Am I correct in assuming you are suggesting mr_pool to limit the number
of MRs as a way to cap MTTE consumption?

However, our goal is to maximize the total registered memory within
the MTTE limits rather than to cap it. In SMC-R, each connection
occupies a configurable, fixed-size registered buffer; consequently,
the more memory we can register, the more concurrent connections
we can support.

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 significantly reduces the total number of entries
required to map the same amount of memory, allowing us to serve more
connections under the same hardware constraints

D. Wythe