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From:   SeongJae Park <sjpark@amazon.com>
To:     Greg Thelen <gthelen@google.com>
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Subject: Re: Re: [PATCH v18 06/14] mm/damon: Implement callbacks for the virtual memory address spaces
Date:   Mon, 27 Jul 2020 11:02:55 +0200
Message-ID: <20200727090255.24114-1-sjpark@amazon.com>
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On Mon, 27 Jul 2020 00:34:54 -0700 Greg Thelen <gthelen@google.com> wrote:

> SeongJae Park <sjpark@amazon.com> wrote:
> 
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > This commit introduces a reference implementation of the address space
> > specific low level primitives for the virtual address space, so that
> > users of DAMON can easily monitor the data accesses on virtual address
> > spaces of specific processes by simply configuring the implementation to
> > be used by DAMON.
[...]
> > diff --git a/mm/damon.c b/mm/damon.c
> > index b844924b9fdb..386780739007 100644
> > --- a/mm/damon.c
> > +++ b/mm/damon.c
> > @@ -9,6 +9,9 @@
[...]
> > +/*
> > + * Functions for the access checking of the regions
> > + */
> > +
> > +static void damon_mkold(struct mm_struct *mm, unsigned long addr)
> > +{
> > +	pte_t *pte = NULL;
> > +	pmd_t *pmd = NULL;
> > +	spinlock_t *ptl;
> > +
> > +	if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> > +		return;
> > +
> > +	if (pte) {
> > +		if (pte_young(*pte)) {
> > +			clear_page_idle(pte_page(*pte));
> > +			set_page_young(pte_page(*pte));
> 
> While this compiles without support for PG_young and PG_idle, I assume
> it won't work well because it'd clear pte.young without setting
> PG_young.  And this would mess with vmscan.

You're right, thanks for catching this up!  This definitely need to be fixed in
the next spin.

> 
> So this code appears to depend on PG_young and PG_idle, which are
> currently only available via CONFIG_IDLE_PAGE_TRACKING.  DAMON could
> depend on CONFIG_IDLE_PAGE_TRACKING via Kconfig.  But I assume that
> CONFIG_IDLE_PAGE_TRACKING and CONFIG_DAMON cannot be concurrently used
> because they'll stomp on each other's use of pte.young, PG_young,
> PG_idle.
> So I suspect we want:
> 1. CONFIG_DAMON to depend on !CONFIG_IDLE_PAGE_TRACKING and vise-versa.
> 2. PG_young,PG_idle and related helpers to depend on
>    CONFIG_DAMON||CONFIG_IDLE_PAGE_TRACKING.

Awesome insights and suggestions, thanks!

I would like to note that DAMON could be interfered by IDLE_PAGE_TRACKING and
vmscan, but not vice versa, as DAMON respects PG_idle and PG_young.  This
design came from the weak goal of DAMON.  DAMON aims to provide not perfect
monitoring but only best effort accuracy that would be sufficient for
performance-centric DRAM level memory management.  So, at that time, I thought
being interfered by IDLE_PAGE_TRACKING and the reclaim logic would not be a
real problem but letting IDLE_PAGE_TRACKING coexist is somehow beneficial.
That said, I couldn't find a real benefit of the coexistance yet, and the
problem of being interference now seems bigger as we will support more cases
including the page granularity.

Maybe we could make IDLE_PAGE_TRACKING and DAMON coexist but mutual exclusive
in runtime, if the beneficial of coexistance turns out big.  However, I would
like to make it simple first and optimize the case later if real requirement
found.

So, I will implement your suggestions in the next spin.  If you have different
opinions, please feel free to comment.


Thanks,
SeongJae Park

> 
> > +		}
> > +		*pte = pte_mkold(*pte);
> > +		pte_unmap_unlock(pte, ptl);
> > +		return;
> > +	}
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +	if (pmd_young(*pmd)) {
> > +		clear_page_idle(pmd_page(*pmd));
> > +		set_page_young(pmd_page(*pmd));
> > +	}
> > +	*pmd = pmd_mkold(*pmd);
> > +	spin_unlock(ptl);
> > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> > +}
> > +
[...]