Re: [LSF/MM/BPF TOPIC] Beyond 2MB: Why Terabyte-Scale Machines Need 1GB Transparent Huge Pages

[Rik van Riel <riel@surriel.com>]

On Thu, 2026-02-19 at 15:53 +0000, Usama Arif wrote:
> 
> Is CMA needed to make this work?
> ================================
> 
> The short answer is no. 1G THPs can be gotten without it. CMA can
> help a lot
> ofcourse, but we dont *need* it. For e.g. I can run the very simple
> case of
> trying to get 1G pages in the upstream kernel without CMA on my
> server via
> hugetlb and it works. The server has been up for more than 2 weeks
> (so pretty
> fragmented), is running a bunch of stuff in the background, uses 0
> CMA memory,
> and I tried to get 100x1G pages on it and it worked.
> It uses folio_alloc_gigantic, which is exactly what this RFC uses:

While I agree with the idea of starting simple, I think
we should ask the question of what we want physical memory
handling to look like if 1TB pages become more common,
and applications start to rely on them to meet their
performance goals.

We have CMA balancing code today. It seems to work, but
it likely is not the long term direction we want to go,
mostly due to the way CMA does allocations.

It seems clear that in order to prevent memory fragmentation,
we need to split up system memory in some way between an area
that is used only for movable allocations, and an area where
any kind of allocation can go.

This would need something similar to CMA balancing to prevent
false OOMs for non-movable allocations.

However, beyond that I really do not have any idea of what
things should look like.

What do we want the kernel to do here?


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