Re: [RFC PATCH 0/4] mm: compaction: mTHP-friendly memory compaction

[Bo Zhang]

On Sun Sep 07, 2026 at 10:51 PM EDT, Zi Yan wrote:
> But skip_isolation_on_order() skips a folio with an order >= target
> order.
> ...
> Oh, you are targeting proactive compaction, where
> skip_isolation_on_order() does not apply.

Right. To clarify the cover letter: the "migrating folios that already
satisfy mTHP" concern is specific to proactive compaction, where
target_order is -1 (via compact_memory) and the order >= target_order
check in skip_isolation_on_order() does not apply. For compaction with an
explicit target order that path already handles it.

> What about the impact on THP compaction? How does it affect direct
> compaction for both mTHP and THP?
>
> It sounds to me that this patch series target proactive compaction. Am I
> getting right?

Two things:

1) Traditional THP (order-9) is not affected. The mTHP-aware branch in
zone_effective_free_pages() only triggers when order == compact_hpage_order(),
i.e. the minimum always-enabled mTHP order (e.g. order-2). An order-9 THP
request does not match that, so it keeps its original behavior exactly
(NR_FREE_PAGES_BLOCKS under defrag_mode, NR_FREE_PAGES otherwise). We
didn't change the THP path.

2) The series isn't limited to proactive compaction. Patches 1-3 target
proactive compaction, but patch 4 also covers the kswapd -> kcompactd path
via pgdat_balanced(), so both proactive compaction and kswapd wakeup are
addressed.

For direct compaction: patch 4 does touch compaction_suit_allocation_order(),
which is shared with direct compaction, so order-2 mTHP direct compaction
would also fall into the new accounting. However, the direct compaction case
needs more testing and thought. How direct compaction should behave for mTHP
is something worth discussing together to decide the right approach.

Thanks for the review.

Bo