linux-kernel - Re: [BUG REPORT] mm/damon: softlockup when kdamond walk page with cpu hotplug

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Message-Id: <20250918085701.59085-1-sj@kernel.org>
Date: Thu, 18 Sep 2025 01:57:00 -0700
From: SeongJae Park <sj@...nel.org>
To: SeongJae Park <sj@...nel.org>
Cc: Xinyu Zheng <zhengxinyu6@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	damon@...ts.linux.dev,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	zouyipeng@...wei.com,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [BUG REPORT] mm/damon: softlockup when kdamond walk page with cpu hotplug

On Thu, 18 Sep 2025 01:52:49 -0700 SeongJae Park <sj@...nel.org> wrote:

> Hello,
> 
> On Thu, 18 Sep 2025 03:00:29 +0000 Xinyu Zheng <zhengxinyu6@...wei.com> wrote:
> 
> > A softlockup issue was found with stress test:
[...]
> damon_mkold_pmd_entry() calls pte_offset_map_lock().  If the call returns an
> error, damon_mkold_pmd_entry() sets walk->action as ACTION_AGAIN, to retry.  If
> the pte_offset_map_lock() continues fails, infinite loop can happen.  I
> understand the loop you mentioned above is this case.
> 
> The error handling (retrying) was introduced by commit 7780d04046a2
> ("mm/pagewalkers: ACTION_AGAIN if pte_offset_map_lock() fails").

I added Hugh to this thread from the original mail of this reply, because he is
the author of the commit.  I should clarified this on the original mail but
forgot saying that, sorry.


Thanks,
SJ

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