Re: [PATCH v8 0/8] mm: Hot page tracking and promotion infrastructure

[Yongting Lin]

On Tue, Jul 28, 2026 at 07:24:46PM +0100, Matthew Wilcox wrote:
> On Tue, Jul 28, 2026 at 11:14:01AM -0700, Andrew Morton wrote:
> > On Tue, 28 Jul 2026 11:13:48 +0530 Bharata B Rao <bharata@xxxxxxx> wrote:
> > 
> > > This patchset introduces pghot, a subsystem for hot page tracking and
> > > promotion.
> > 
> > Can DAMON be used to do this sort of thing adequately?
> > 
> > Thanks, I'll await review input on this.
> 
> I don't think we should do this.  I don't think it's a use-case that
> customers actually want; I think it's something CPU vendors think their
> customers want.  I've said this repeatedly over the months this patchset
> has been in development.  I say no.
>

>From our perspective, this is not merely a hypothetical use case. We
are actively developing and evaluating CXL tiered-memory systems, and
expect promoting CXL-resident pages that become hot back to DRAM to be
a practical requirement.

NUMA balancing mode 2 already provides such a promotion path. What we
are looking for is a better hot-page detection mechanism than NUMA hint
faults. AMD IBS is an interesting option, but the v8 results do not yet
show a clear performance advantage over hint faults.

IBS is not the only possible hardware source. CXL 3.2 defines CHMU for
device-side hotness monitoring, and CHMU has also been discussed in the
DAMON and CXL hotness-monitoring discussions.

CHMU does not normally provide the CPU NUMA node that generated an
access, which complicates target-node selection on multi-socket systems.
However, single-socket systems are also common across data-center CPU
generations. On such systems, the missing requester NUMA node is much
less of a limitation.

This suggests that a configurable framework capable of consuming
different hotness sources, depending on the platform topology and
available hardware, would be useful.

We would also be interested in helping test and compare the different
approaches as implementations become available.

Thanks,
Yongting Lin