Re: [RFC PATCH 0/4] Extend xas_split* to support splitting arbitrarily large entries

[Matthew Wilcox <willy@infradead.org>]

On Mon, Aug 24, 2026 at 10:47:55PM +0300, Artem Lytkin wrote:
> I'm asking because there is a second user for it that has nothing to do
> with guest_memfd. On arm64 with 64K pages HPAGE_PMD_ORDER is 13 and
> MAX_XAS_ORDER is 11, so MAX_PAGECACHE_ORDER clamps to 11 and shmem never
> offers a PMD order. MADV_COLLAPSE on tmpfs returns -EINVAL there, and
> huge=always hands out order-11 folios at best, so ShmemPmdMapped never
> leaves zero. Every other page cache file system tops out at order 11 as
> well, which is 128M and not PMD-mappable. pagemap.h admits it outright:
> "xas_split_alloc() does not support arbitrary orders. This implies no
> 512MB THP on ARM64 with 64KB base page size." Kiryl's khugepaged
> selftest series skips the shmem and file cases altogether for exactly
> this reason, and on 6 August he confirmed in that thread that uniform
> splits still go through xas_split_alloc(), so xas_try_split() on its
> own does not lift the cap.

But why would you want it?  With 64KiB pages on arm64, you need 512MiB
for a PMD-sized page, and that's just too big for current hardware.
What's your bandwidth?  40GB per DIMM?  With 8 DIMMs, that's an
aggregate of 320GB bandwidth, so that restricts you to 640 page faults
per second.  That's too few for almost all workloads.

The page allocator also isn't great at doing order-13 allocations.

Since arm64 does support 2MiB TLBs, even with a 512MiB PMD size, I'm
not sure there's a huge performance win waiting for you on the other
side of all this work.

IOW is this driven by a real need, or just some completionist thing?

> That case needs two new levels of nodes rather than one: 130 nodes,
> 73 KiB, a long way from the 1G to 4K split Matthew was worried about.
> Raising MAX_XAS_ORDER to 3 * XA_CHUNK_SHIFT - 1 covers it, and
> MAX_PAGECACHE_ORDER stays min()'d with HPAGE_PMD_ORDER, so no page cache
> caller can ask for the order-17 worst case that cap admits.

130 nodes is still a lot to ask the slab allocator for!  With 64KiB
pages it's not too bad -- you get 112 per page.  But still ...