Re: [RFC PATCH v3 05/37] mm/{mprotect,memory}: (no upstream-aimed hack) implement MM_CP_DAMON

[SeongJae Park <sj@kernel.org>]

+ damon@

On Mon, 8 Dec 2025 12:19:41 +0100 "David Hildenbrand (Red Hat)" <david@kernel.org> wrote:

> On 12/8/25 07:29, SeongJae Park wrote:
> > Note that this is not upstreamable as-is.  This is only for helping
> > discussion of other changes of its series.
> > 
> > DAMON is using Accessed bits of page table entries as the major source
> > of the access information.  It lacks some additional information such as
> > which CPU was making the access.  Page faults could be another source of
> > information for such additional information.
> > 
> > Implement another change_protection() flag for such use cases, namely
> > MM_CP_DAMON.  DAMON will install PAGE_NONE protections using the flag.
> > To avoid interfering with NUMA_BALANCING, which is also using PAGE_NON
> > protection, pass the faults to DAMON only when NUMA_BALANCING is
> > disabled.
> > 
> > Again, this is not upstreamable as-is.  There were comments about this
> > on the previous version, and I was unable to take time on addressing
> > those.  As a result, this version is not addressing any of those
> > previous comments.  I'm sending this, though, to help discussions on
> > patches of its series, except this one.  Please forgive me adding this
> > to your inbox without addressing your comments, and ignore.  I will
> > establish another discussion for this part later.
> > 
> > Signed-off-by: SeongJae Park <sj@kernel.org>
> > ---
[...]
> > @@ -6363,8 +6415,12 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
> >   		return 0;
> >   	}
> >   	if (pmd_trans_huge(vmf.orig_pmd)) {
> > -		if (pmd_protnone(vmf.orig_pmd) && vma_is_accessible(vma))
> > +		if (pmd_protnone(vmf.orig_pmd) && vma_is_accessible(vma)) {
> > +			if (sysctl_numa_balancing_mode ==
> > +					NUMA_BALANCING_DISABLED)
> > +				return do_damon_page(&vmf, true);
> >   			return do_huge_pmd_numa_page(&vmf);
> > +		}
> 
> I recall that we had a similar discussion already. Ah, it was around 
> some arm MTE tag storage reuse [1].
> 
> The idea was to let do_*_numa_page() handle the restoring so we don't 
> end up with such duplicated code.

Thank you for sharing this, David!  As I mentioned on the commit description, I
will revisit this part for making this more upstreamable, after LPC.  And this
previous conversation will be really useful for me at preparing the next
iteration.  Thank you again!

> 
> [1] 
> https://lore.kernel.org/all/20240125164256.4147-1-alexandru.elisei@arm.com/


Thanks,
SJ

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