Re: [PATCH net-next 2/3] mm: vmalloc: export find_vm_area()

Uladzislau Rezki
2026-01-26T10:28:51Z

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@linux.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.
>
> 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!
>
Appreciate for the explanation. Yes it clarifies an intention.

As for proposed patch above:

- A page_order is available if CONFIG_HAVE_ARCH_HUGE_VMALLOC is defined;
- It makes sense to get a node, grab a spin-lock and find VM, save
page_order and release the lock.

You can have a look at the vmalloc_dump_obj(void *object) function.
We try-spinlock there whereas you need just spin-lock. But the idea
is the same.

--
Uladzislau Rezki