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

["David Hildenbrand (Arm)" <david@kernel.org>]

> 
> 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.

However, we cannot easily do the same when remapping a 1 GiB THP to be 
mapped by PMDs etc. I think there are examples where that just doesn't 
work or is not desired.

But I considered that in general (avoid folio_split()) an interesting 
approach. The remapping part is a bit different though.

-- 
Cheers,

David