Re: [PATCH v7 0/7] mm: Hot page tracking and promotion infrastructure

[Bharata B Rao]

On 06-May-26 8:52 PM, Gregory Price wrote:
> On Mon, May 04, 2026 at 09:36:05PM +0100, Matthew Wilcox wrote:
>> On Mon, May 04, 2026 at 11:39:17AM +0530, Bharata B Rao wrote:
>>> This is v7 of pghot, a hot-page tracking and promotion subsystem. The
>>
>> I continue to think we should not do this.
> 
> My only pushback on the general "we should not do this" is that we need
> something to counter-balance the demotion bit in vmscan.c, and the
> current implementation (prot_none faults) is rather :[

So you are saying pghot subsystem currently does hot page detection and
promotion only, which is fine. But the current implementation of demotion is not
very optimal and hence we should spend effort in fine-tuning demotion first?

In this series itself I have shown via benchmark numbers that for over-committed
cases (involving both demotion and promotion), the workload isn't really showing
real benefit due to demotion and promotion. Are you specifically referring to
this problem?

> 
> I think this series needs to greatly limit its complexity and provide
> some gentle correction for LRU inversions, and I think they're making a
> decent attempt at that.

Regarding complexity, I agree that the initial version of this patchset was
quite complicated in the way it maintained hot page information. But the later
versions including this one have greatly reduced the complexity with one byte of
hot page information per PFN, atomic updates to hotness data without any locks,
per-lowertier kmigrated threads for promotion and reuse of existing hot page
promotion engine.

Did you have anything else in mind wrt complexity?

Can you provide more context about the LRU inversion problem?

Regards,
Bharata.