Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time

[David Hildenbrand (Arm)]

On 2/11/26 20:28, Matthew Wilcox wrote:
> On Wed, Feb 11, 2026 at 04:49:45AM -0800, Usama Arif wrote:
> > - Memory reclaim (try_to_unmap()): Returns false, folio rotated back
> > LRU, retried in next reclaim cycle.
>
> I was advised to ask my stupid question ...
>
> Why do we still try to split the PMD in reclaim?  I understand we're
> about to swap the folio out and we'll need to put a swap entry in the page
> table so we can find it again.  But can't we now store swap entries at the
> PMD level, or are we still forced to store 512 entries at the PTE level?

Yes. We don't support PMD swap entries yet.

I don't know all historical details. I suspect there are some rough 
edges around swapin (assume we cannot swapin a 2M THP), and maybe it was 
just easier to not deal with splitting of PMD swap entries (which we 
would similarly have to support).

For sure an interesting project to look into.

--
Cheers,

David