Re: [LSF/MM/BPF TOPIC] 64k (or 16k) base page size on x86

[Zi Yan <ziy@nvidia.com>]

On 19 Feb 2026, at 11:09, David Hildenbrand (Arm) wrote:

> On 2/19/26 16:54, Kiryl Shutsemau wrote:
>> On Thu, Feb 19, 2026 at 04:39:34PM +0100, David Hildenbrand (Arm) wrote:
>>> On 2/19/26 16:08, Kiryl Shutsemau wrote:
>>>> No, there's no new hardware (that I know of). I want to explore what page size
>>>> means.
>>>>
>>>> The kernel uses the same value - PAGE_SIZE - for two things:
>>>>
>>>>     - the order-0 buddy allocation size;
>>>>
>>>>     - the granularity of virtual address space mapping;
>>>>
>>>> I think we can benefit from separating these two meanings and allowing
>>>> order-0 allocations to be larger than the virtual address space covered by a
>>>> PTE entry.
>>>>
>>>> The main motivation is scalability. Managing memory on multi-terabyte
>>>> machines in 4k is suboptimal, to say the least.
>>>>
>>>> Potential benefits of the approach (assuming 64k pages):
>>>>
>>>>     - The order-0 page size cuts struct page overhead by a factor of 16. From
>>>>       ~1.6% of RAM to ~0.1%;
>>>>
>>>>     - TLB wins on machines with TLB coalescing as long as mapping is naturally
>>>>       aligned;
>>>>
>>>>     - Order-5 allocation is 2M, resulting in less pressure on the zone lock;
>>>>
>>>>     - 1G pages are within possibility for the buddy allocator - order-14
>>>>       allocation. It can open the road to 1G THPs.
>>>>
>>>>     - As with THP, fewer pages - less pressure on the LRU lock;
>>>>
>>>>     - ...
>>>>
>>>> The trade-off is memory waste (similar to what we have on architectures with
>>>> native 64k pages today) and complexity, mostly in the core-MM code.
>>>>
>>>> == Design considerations ==
>>>>
>>>> I want to split PAGE_SIZE into two distinct values:
>>>>
>>>>     - PTE_SIZE defines the virtual address space granularity;
>>>>
>>>>     - PG_SIZE defines the size of the order-0 buddy allocation;
>>>>
>>>> PAGE_SIZE is only defined if PTE_SIZE == PG_SIZE. It will flag which code
>>>> requires conversion, and keep existing code working while conversion is in
>>>> progress.
>>>>
>>>> The same split happens for other page-related macros: mask, shift,
>>>> alignment helpers, etc.
>>>>
>>>> PFNs are in PTE_SIZE units.
>>>>
>>>> The buddy allocator and page cache (as well as all I/O) operate in PG_SIZE
>>>> units.
>>>>
>>>> Userspace mappings are maintained with PTE_SIZE granularity. No ABI changes
>>>> for userspace. But we might want to communicate PG_SIZE to userspace to
>>>> get the optimal results for userspace that cares.
>>>>
>>>> PTE_SIZE granularity requires a substantial rework of page fault and VMA
>>>> handling:
>>>>
>>>>     - A struct page pointer and pgprot_t are not enough to create a PTE entry.
>>>>       We also need the offset within the page we are creating the PTE for.
>>>>
>>>>     - Since the VMA start can be aligned arbitrarily with respect to the
>>>>       underlying page, vma->vm_pgoff has to be changed to vma->vm_pteoff,
>>>>       which is in PTE_SIZE units.
>>>>
>>>>     - The page fault handler needs to handle PTE_SIZE < PG_SIZE, including
>>>>       misaligned cases;
>>>>
>>>> Page faults into file mappings are relatively simple to handle as we
>>>> always have the page cache to refer to. So you can map only the part of the
>>>> page that fits in the page table, similarly to fault-around.
>>>>
>>>> Anonymous and file-CoW faults should also be simple as long as the VMA is
>>>> aligned to PG_SIZE in both the virtual address space and with respect to
>>>> vm_pgoff. We might waste some memory on the ends of the VMA, but it is
>>>> tolerable.
>>>>
>>>> Misaligned anonymous and file-CoW faults are a pain. Specifically, mapping
>>>> pages across a page table boundary. In the worst case, a page is mapped across
>>>> a PGD entry boundary and PTEs for the page have to be put in two separate
>>>> subtrees of page tables.
>>>>
>>>> A naive implementation would map different pages on different sides of a
>>>> page table boundary and accept the waste of one page per page table crossing.
>>>> The hope is that misaligned mappings are rare, but this is suboptimal.
>>>>
>>>> mremap(2) is the ultimate stress test for the design.
>>>>
>>>> On x86, page tables are allocated from the buddy allocator and if PG_SIZE
>>>> is greater than 4 KB, we need a way to pack multiple page tables into a
>>>> single page. We could use the slab allocator for this, but it would
>>>> require relocating the page-table metadata out of struct page.
>>>
>>> When discussing per-process page sizes with Ryan and Dev, I mentioned that
>>> having a larger emulated page size could be interesting for other
>>> architectures as well.
>>>
>>> That is, we would emulate a 64K page size on Intel for user space as well,
>>> but let the OS work with 4K pages.
>>>
>>> We'd only allocate+map large folios into user space + pagecache, but still
>>> allow for page tables etc. to not waste memory.
>>>
>>> So "most" of your allocations in the system would actually be at least 64k,
>>> reducing zone lock contention etc.
>>
>> I am not convinced emulation would help zone lock contention. I expect
>> contention to be higher if page allocator would see a mix of 4k and 64k
>> requests. It sounds like constant split/merge under the lock.
>
> If most your allocations are larger, then there isn't that much splitting/merging.
>
> There will be some for the < 64k allocations of course, but when all user space+page cache is >= 64 then the split/merge + zone lock should be heavily reduced.
>
>>
>>> It doesn't solve all the problems you wanted to tackle on your list (e.g.,
>>> "struct page" overhead, which will be sorted out by memdescs).
>>
>> I don't think we can serve 1G pages out of buddy allocator with 4k
>> order-0. And without it, I don't see how to get to a viable 1G THPs.
>
> Zi Yan was one working on this, and I think we had ideas on how to make that work in the long run.

Right. The idea is to add super pageblock (or whatever name), which consists of N consecutive
pageblocks, so that anti fragmentation can work at larger granularity, e.g., 1GB, to create
free pages. Whether 1GB free pages from memory compaction need to go into buddy allocator
or not is debatable.

--
Best Regards,
Yan, Zi