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

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Message-ID: <20260127133417.GU13967@unreal>
Date: Tue, 27 Jan 2026 15:34:17 +0200
From: Leon Romanovsky <leon@...nel.org>
To: "D. Wythe" <alibuda@...ux.alibaba.com>
Cc: 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 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

> 
> To address this, we intend to use vmalloc_huge(). When it successfully
> allocates high-order pages, the vmalloc area is backed by a sequence of
> physically contiguous chunks (e.g., 2MB each). If we know this
> page_order, we can register these larger physical blocks instead of
> individual 4KB pages, reducing MTT consumption from 512 entries down to
> 1 for every 2MB of memory (with page_order == 9).
> 
> However, the result of vmalloc_huge() is currently opaque to the caller.
> We cannot determine whether it successfully allocated huge pages or fell
> back to 4KB pages based solely on the returned pointer. Therefore, we
> need a helper function to query the actual page order, enabling SMC-R to
> adapt its registration logic to the underlying physical layout.
> 
> I hope this clarifies our design motivation!
> 
> Best regards,
> D. Wythe
> 
> 
> 
> 
>