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Wythe" Cc: Uladzislau Rezki , "David S. Miller" , Andrew Morton , Dust Li , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Sidraya Jayagond , Wenjia Zhang , Mahanta Jambigi , Simon Horman , Tony Lu , Wen Gu , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-rdma@vger.kernel.org, linux-s390@vger.kernel.org, netdev@vger.kernel.org, oliver.yang@linux.alibaba.com Subject: Re: [PATCH net-next 2/3] mm: vmalloc: export find_vm_area() Message-ID: References: <20260123082349.42663-1-alibuda@linux.alibaba.com> <20260123082349.42663-3-alibuda@linux.alibaba.com> <20260124093505.GA98529@j66a10360.sqa.eu95> <20260124145754.GA57116@j66a10360.sqa.eu95> Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260124145754.GA57116@j66a10360.sqa.eu95> 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 > > > > > --- > > > > > 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