linux-kernel - [Question] Should we reuse target when damon's operation changed?

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Date:   Wed, 30 Nov 2022 15:01:29 +0900
From:   Yun Levi <ppbuk5246@...il.com>
To:     SeongJae Park <sj@...nel.org>
Cc:     damon@...ts.linux.dev, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [Question] Should we reuse target when damon's operation changed?

Hello SJ.

While I try to use damon, I have some questions whether it is correct
to reuse damon_target structure when damon's operation is changed.

At first, one user set up damon like below.
    echo 1 > /sys/kernel/mm/damon/admin/kdamonds/nr_kdamonds
    echo 1 > /sys/kernel/mm/damon/admin/kdamonds/0/contexts/nr_contexts
    echo 1 > /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/nr_targets
    echo 1 > /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/pid_target
    echo 1 > /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/regions/nr_regions
    echo 0 > /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/regions/nr_regions/0/start
    echo 16384 >
/sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/regions/nr_regions/0/end

    echo vaddr >  /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/operation
    echo on >  /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/state

And some time pass, user change the operation as "paddr" like below:
    echo paddr >  /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/operation
    echo commit >  /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/state

In that situation, the damon_target is reused and the region isn't changed,
former region information which damon_region has -- nr_accesse,
last_nr_accesss, last_sample addr is kept.

But, former accessed information is based on vaddr and changed
accessed information should be based on paddr and it seems the wrong
information to new applied operation.

IIUC, it makes some confusion to kdamond when it merges or splits
regions based on above information.

So, Is it much better to remove the target and region information when
the operation is changed? or should we check whether it's possible to
reuse former access information between former and new operation?

Thanks.

-- 
Best regards,
Levi