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

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Feb 19, 2026 at 11:52:57AM -0500, Zi Yan wrote:
> On 19 Feb 2026, at 11:48, Johannes Weiner wrote:
> 
> > On Thu, Feb 19, 2026 at 05:00:19PM +0100, David Hildenbrand (Arm) wrote:
> >>
> >>>
> >>> I see 1G THPs being opportunistically used ideally at the start of the application
> >>> or by the allocator (jemalloc/tcmalloc) when there is plenty of free memory
> >>> available and a greater chance of getting 1G THPs.
> >>>
> >>> Splitting strategy
> >>> ==================
> >>>
> >>> When PUD THP must be break -- for COW after fork, partial munmap, mprotect on
> >>> a subregion, or reclaim -- it splits directly from PUD to PTE level, converting
> >>> 1 PUD entry into 262,144 PTE entries. The ideal solution would be to split to
> >>> PMDs and only the necessary PMDs to PTEs. This is something that would hopefully
> >>> be possible with Davids proposal [3].
> >>
> >> There once was this proposal where we would, instead of splitting a THP,
> >> migrate all memory away instead. That means, instead of splitting the 1
> >> GiB THP, you would instead return it to the page allocator where
> >> somebody else could use it.
> >
> > With TLB coalescing, there is benefit in preserving contiguity. If you
> > lop off the last 4k of a 2M-backed range, a split still gives you 511
> > contiguously mapped pfns that can be coalesced.
> 
> Which CPU are you referring to? AMD’s PTE coalescing works up to 32KB
> and ARM’s contig PTE supports larger sizes. BTW, do we have PMD level
> ARM contiguous bit support?

I'm not aware of a CPU that will coalesce the 511 entries into a
single one. But *any* coalescing effects will be lost when the range
is scattered into discontiguous 4k pagelets.