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

Uladzislau Rezki
2026-01-26T16:45:06Z

On Mon, Jan 26, 2026 at 08:02:26PM +0800, D. Wythe wrote:
> On Mon, Jan 26, 2026 at 11:28:46AM +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@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
>
> Hi Uladzislau,
>
> Thanks very much for the detailed guidance, especially on the correct
> locking pattern. This is extremely helpful.I will follow it and send
> a v2 patch series with the new helper implemented in mm/vmalloc.c.
>
> Thanks again for your support.
>
Welcome!

--
Uladzislau Rezki