Re: [LSF/MM/BPF TOPIC] DAMON Requirements for Access-aware MM of Future

[SeongJae Park <sj@kernel.org>]

On Thu, 2 Jan 2025 10:00:19 -0800 SeongJae Park <sj@kernel.org> wrote:

> On Thu, 2 Jan 2025 10:22:14 -0500 Gregory Price <gourry@gourry.net> wrote:
> 
> > On Thu, Jan 02, 2025 at 04:09:38AM +0000, Matthew Wilcox wrote:
> > > On Wed, Jan 01, 2025 at 02:20:39PM -0800, SeongJae Park wrote:
> > > > Hi all,
> > > > 
> > > > 
> > > > I find a few interesting and promising projects that aim to do efficient access
> > > > pattern-aware memory management of near future, including below (alphabetically
> > > > sorted).
> > > > 
> > > > - CXL hotness monitoring unit
> > > >   (https://lore.kernel.org/20241121101845.1815660-1-Jonathan.Cameron@huawei.com)
> > > > - Memory tiering fainess by per-cgroup control of promotion and demotion
> > > >   (https://lore.kernel.org/20241108190152.3587484-1-kaiyang2@cs.cmu.edu)
> > > > - Promotion of unmapped page cache folios
> > > >   (https://lore.kernel.org/20241210213744.2968-1-gourry@gourry.net)
> > > 
> > > I'm not sure how DAMON can help with this one.  As I understand DAMON,
> > > it monitors accesses to user addresses.  This patchset is trying to solve
> > > the problem for file pages which aren't mapped to userspace at all.
> > > ie only accessed through read() and write().
> > 
> > DAMON can monitor physical addresses to, though the mechanism is
> > different.
> 
> Thank you for answering this, Gregory.  As Gregory explained, users can use
> physical address monitoring mode of DAMON for this.  For unmapped pages, DAMON
> sets and reads PG_idle to check if it is accessed or not.  Since PG_idle is
> respected by read() and write() use case to my understanding, DAMON should be
> able to check accesses to unmapped pages.
> 
> > I haven't assessed this as a solution, yet.
> 
> To quickly see this, I ran below simple test.
[...]
> the point of this test is that DAMON at least somehow react to accesses for
> unmapped pages.

Forgot clarifying this point, sorry.  My test shows DAMON can detect accesses
to unmapped pages, but not asseses if it is feasible as the unmapped pages
promotion solution.  More works and discussions would be needed for that.


Thanks,
SJ

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