Re: [RFC PATCH v3 0/3] mm/damon: introduce DAMOS_SPLIT action

[Lian Wang <lianux.mm@gmail.com>]

Hi Asier,

Thanks for the quick feedback.

On 7/20/2026 12:28 PM, Gutierrez Asier wrote:
> You should mention why page split is be needed. The fact that page
> collapsing exist doesn't necessarily mean that split should exist.

Fair point.  The underlying problem I'm trying to address is that
DAMON's vaddr monitoring loses accuracy under PMD-mapped THP:
multiple sampled addresses share a single Accessed bit, so the
observed hot set is coarser than the true working set.  Split is one
way to restore fine-grain monitoring -- by dismantling the PMD
mapping, each base page gets its own PTE Accessed bit and DAMON can
see the real access distribution again.

Split is not the only possible approach, and it is certainly not
intended to be "the opposite of collapse".  It is just one concrete
proposal to start the discussion.  What I really care about is
whether the community agrees that this monitoring granularity problem
is worth solving.  If there are better ways to address it, I'm very
open to that direction.

The RFC is as much about the problem as it is about the mechanism.
Feedback on real workloads that suffer from this coarsening, and on
alternative approaches, is exactly what I'm hoping for.

> Could you add v1 as well?

Good catch, will add in the next revision.

[1] https://lore.kernel.org/20260620203915.82947-1-sj@kernel.org/

Thanks,
Lian Wang