Re: 5 gigs of contiguos kernel VM shared with userspace

On Sun, 9 Aug 2026 19:15:59 -0400
Mark Johnston <markj@freebsd.org> wrote:

> On Mon, Aug 10, 2026 at 01:32:02AM +0300, Vadim Goncharov wrote:
> > Hello,
> > 
> > Are there ways to allocate in kernel a range of contiguous virtual address
> > space, share it with userspace, and preferably run memory allocator inside
> > it which has it's pointers relative to base address? Same as processes
> > cooperating via mmap(), but for kernel<->user.
> > 
> > I'd like just run UMA inside it, but seems UMA zones cannot be told to take
> > from particular range, or am I missing something? On user side, jemalloc
> > mentions something like arenas, but seems it is too unprepared for even
> > mmap() between processes case (may be adapting some TLSF allocator [*] is
> > possible, but that's a premature question before VM one).  
> 
> UMA is really two fairly distinct things: a slab allocator, and a cache.
> UMA zones provide per-CPU and per-NUMA domain caches, and UMA kegs
> allocate and manage slabs.
> 
> You can have a zone without a keg: if you have some collection of
> objects you want to allocate to consumers, you can use
> uma_zcache_create() to create a "cache zone" which provides the usual
> per-CPU etc. caches to store objects returned by a custom uz_import
> callback that you provide when creating the zone.  For instance, if you
> have some static array of objects (i.e., there is no need to subdivide a
> slab) and just want to slap a scalable cache in front of it,
> uma_zcache_create() is handy.
> 
> The other thing you could do is provide a custom slab allocator for the
> keg via uma_zone_set_allocf().  There, you provide some page allocator,
> and you can impose whatever constraints you want.  uma_small_alloc() is
> used by default for page-sized slabs, and page_alloc() is used by

Is that all ported/portable to userspace?

> default for multi-page slabs.  Your implementation could return pages
> from some pre-allocated, contiguous range of memory.

That's the main [prerequisite] question: how to do in kernel thing similar
to mmap(0, 0x5000000, MAP_SHARED|MAP_ALIGNED_SUPER, ...) and share this
region with userspace process(es).

> > Size is not necessarily gigabytes, I expect it more often to be just
> > megabytes (see ARB_* family of macros/functions), but in future can be. If
> > it ever can be turned to CoW transactional semantics (think of SQLite
> > journal, but in memory), it would be wonderful!
> > 
> > 
> > [*] A paper and several implementations:
> > http://www.gii.upv.es/tlsf/files/papers/ecrts04_tlsf.pdf
> > https://github.com/mattconte/tlsf
> > https://github.com/sysprog21/tlsf-bsd
> > https://github.com/rmind/tlsf
> > 
> > -- 
> > WBR, @nuclight
> >   



-- 
WBR, @nuclight