From: SeongJae Park <sj@kernel.org>
To: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
Cc: SeongJae Park <sj@kernel.org>,
damon@lists.linux.dev, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
akpm@linux-foundation.org, corbet@lwn.net, bijan311@gmail.com,
ajayjoshi@micron.com, honggyu.kim@sk.com, yunjeong.mun@sk.com
Subject: Re: [RFC PATCH 1/5] mm/damon/core: fix nr_accesses_bp underflow in damon_moving_sum
Date: Sun, 17 May 2026 11:16:25 -0700 [thread overview]
Message-ID: <20260517181626.4104-1-sj@kernel.org> (raw)
In-Reply-To: <20260516210357.2247-2-ravis.opensrc@gmail.com>
Hello Ravi,
On Sat, 16 May 2026 14:03:53 -0700 Ravi Jonnalagadda <ravis.opensrc@gmail.com> wrote:
> Guard against unsigned integer underflow when nomvsum/len_window
> exceeds mvsum.
How could this happen? mvsum is assumed to be same to nomvsum at the beginning
of the window. Hence, even if there is only zero new_value, at the end of the
window, mvsum should be exactly zero. Of course there could be a bug that
breaks the assumption.
> When that subtraction wraps, the moving sum returns a
> near-ULONG_MAX value and corrupts nr_accesses_bp.
>
> If subtrahend > mvsum, return new_value: this clamps the moving-sum
> estimate to the current observation rather than wrapping.
I guess you saw this issue in real, and this change should fix the issue. But
I think we should know why and how mvsum < nomvum / len_window can unexpectedly
happen, and fix that.
Could you share more details about when and how the situation happens?
Thanks,
SJ
[...]
next prev parent reply other threads:[~2026-05-17 18:16 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-16 21:03 [RFC PATCH 0/5] mm/damon: DAMOS quota controller and paddr migration walk fixes Ravi Jonnalagadda
2026-05-16 21:03 ` [RFC PATCH 1/5] mm/damon/core: fix nr_accesses_bp underflow in damon_moving_sum Ravi Jonnalagadda
2026-05-17 18:16 ` SeongJae Park [this message]
2026-05-16 21:03 ` [RFC PATCH 2/5] mm/damon/core: cap effective quota size to total monitored memory Ravi Jonnalagadda
2026-05-17 18:36 ` SeongJae Park
2026-05-16 21:03 ` [RFC PATCH 3/5] mm/damon/core: floor effective quota size at minimum region size Ravi Jonnalagadda
2026-05-17 18:47 ` SeongJae Park
2026-05-16 21:03 ` [RFC PATCH 4/5] mm/damon/paddr: skip free pageblocks in migration walk Ravi Jonnalagadda
2026-05-16 21:03 ` [RFC PATCH 5/5] mm/damon/paddr: add time budget to migration page walk Ravi Jonnalagadda
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