Re: [PATCH v2 0/2] mm/damon: reset thread status parameters upon kdamond termination

SeongJae Park
2026-04-14T00:34:46Z

On Mon, 13 Apr 2026 17:23:03 -0700 SeongJae Park <sj@kernel.org> wrote:

> On Tue, 14 Apr 2026 02:52:47 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:
>
> > Problem
> > ========
>
> Let's align the underline with the subject. Also, let's add one blank line
> after the underline.
>
> > When kdamond terminates unexpectedly, 'enabled' remains 'Y' and
> > 'kdamond_pid' remains stale. This prevents user from restarting DAMON
> > because both writing 'Y' and 'N' to 'enabled' will fail.
> >
> > "Unexpected termination" here means the kdamond exits without any user
> > request (e.g., not by writing 'N' to 'enabled').
>
> Could you please further explain when such termination can happen?
>
> >
> > User Impact
> > ===========
> > Once kdamond terminates this way, it cannot be restarted via sysfs
> > because:
> >
> > 1. DAMON_LRU_SORT/DAMON_RECLAIM is built into the kernel, so it cannot
> > be unloaded and reloaded at runtime.
>
> I think this is quite obvious, so may better to be dropped.
>
> > 2. Writing 'N' to 'enabled' fails because kdamond no longer exists;
> > Writing 'Y' does nothing, as 'enabled' is already Y.
> >
> > As a result, the only way to restore DAMON functionality is a full
> > system reboot.
>
> Thank you for clarifying the user impact. I think this deserves Cc-ing
> stable@.
>
> I think 'Problem' and 'User Impact' can be unified into one section.
>
> >
> > Solution
> > ========
> > damon_commit_ctx() sets 'maybe_corrupted=true' at the beginning and only
> > sets it to false upon successful completion. When 'maybe_corrupted'
> > remains true, kdamond will terminate eventually.
>
> This seems better to be explained earlier, on the problem section.
>
> >
> > Therefore:
> > 1. In damon_{lru_sort, reclaim}_turn(): Add fallback logic to reset
> > parameters when damon_stop() fails but kdamond is not running.
> > 2. In damon_{lru_sort, reclaim}_apply_parameters(): Reset parameters
> > when damon_commit_ctx() fails, as kdamond will terminate due to
> > maybe_corrupted mechanism.
>
> So the problem is that 'enable' parameter value is not trustworthy, and this
> series is trying to make it trustworthy. I think it is bit complicated,
> especially for stable@ fix. What about simply using more trustworthy
> information, e.g.,
>
> '''
> --- a/mm/damon/reclaim.c
> +++ b/mm/damon/reclaim.c
> @@ -390,7 +390,7 @@ MODULE_PARM_DESC(addr_unit,
> static int damon_reclaim_enabled_store(const char *val,
> const struct kernel_param *kp)
> {
> - bool is_enabled = enabled;
> + bool is_enabled = false;
> bool enable;
> int err;
>
> @@ -398,6 +398,9 @@ static int damon_reclaim_enabled_store(const char *val,
> if (err)
> return err;
>
> + if (ctx)
> + is_enabled = damon_is_running(ctx);
> +
> if (is_enabled == enable)
> return 0;
>
> '''
>
> >
> > Changes from RFC-v1
> > (https://lore.kernel.org/20260330164347.12772-1-aethernet65535@gmail.com)
> > - Remove RFC tag.
>
> When dropping RFC tag, let's start from v1 again, from the next time.

Also, I just found the patches don't have Fixes: and Cc: stable@. Could you
please add those appripriately?

Thanks,
SJ

>
>
> Thanks,
> SJ
>
> [...]
>