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

[SeongJae Park <sj@kernel.org>]

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.


Thanks,
SJ

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