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Thu, 03 Sep 2026 03:01:50 -0700 (PDT) From: Kunwu Chan X-Google-Original-From: Kunwu Chan To: SJ Park Cc: Kunwu Chan , Andrew Morton , stable@vger.kernel.org, Quanmin Yan , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Kunwu Chan Subject: Re: [PATCH 4/4] mm/damon/sysfs: set next refresh jiffies per sysfs context Date: Thu, 3 Sep 2026 18:01:41 +0800 Message-ID: <20260903100142.106449-1-kunwu.chan@linux.dev> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260902002725.108635-5-sj@kernel.org> References: Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Tue, 1 Sep 2026 17:27:23 -0700 SJ Park wrote: > When 'refresh_ms' is set, DAMON sysfs interface periodically updates > auto-tuned parameters and DAMOS stats. The timestamp for the next > refresh is initialized when a DAMON context starts, and updated in its > damon_call() callback function. That is, each DAMON context updates it. > However, the timestamp is a global variable that is shared with all the > contexts. When there are multiple DAMON contexts having different > refresh_ms, the update frequency will be changed, depending on the order > of the contexts. When there are multiple kdamonds, it will be even more > chaotic. Fix the problem by having the timestamp per each context. > > The user impact is not very critical. It does not leak, corrupt or > crash. The update will not be faster or slower than the lowest and > largest refresh_ms values of the contexts, respectively. The user can > also manually ask the updates on demand using kdamond state commands. > That said, clearly this is a bug and can easily be reproduced. > > Fixes: 9fd7bb5083d1 ("mm/damon/sysfs: change next_update_jiffies to a global variable") > Cc: # 6.18.x > Signed-off-by: SJ Park > --- > mm/damon/sysfs.c | 11 +++++------ > 1 file changed, 5 insertions(+), 6 deletions(-) > > diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c > index 3c81b4c91ac0d..dcb739ce0a729 100644 > --- a/mm/damon/sysfs.c > +++ b/mm/damon/sysfs.c > @@ -1788,6 +1788,7 @@ struct damon_sysfs_kdamond { > struct damon_sysfs_contexts *contexts; > struct damon_ctx *damon_ctx; > unsigned int refresh_ms; > + unsigned long next_refresh_jiffies; > }; > > static struct damon_sysfs_kdamond *damon_sysfs_kdamond_alloc(void) > @@ -2213,17 +2214,15 @@ static struct damon_ctx *damon_sysfs_build_ctx( > return ctx; > } > > -static unsigned long damon_sysfs_next_update_jiffies; > - > static int damon_sysfs_repeat_call_fn(void *data) > { > struct damon_sysfs_kdamond *sysfs_kdamond = data; > > if (!sysfs_kdamond->refresh_ms) > return 0; > - if (time_before(jiffies, damon_sysfs_next_update_jiffies)) > + if (time_before(jiffies, sysfs_kdamond->next_refresh_jiffies)) > return 0; > - damon_sysfs_next_update_jiffies = jiffies + > + sysfs_kdamond->next_refresh_jiffies = jiffies + > msecs_to_jiffies(sysfs_kdamond->refresh_ms); > > if (!mutex_trylock(&damon_sysfs_lock)) > @@ -2271,8 +2270,8 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond) > } > kdamond->damon_ctx = ctx; > > - damon_sysfs_next_update_jiffies = > - jiffies + msecs_to_jiffies(kdamond->refresh_ms); > + kdamond->next_refresh_jiffies = jiffies + > + msecs_to_jiffies(kdamond->refresh_ms); The fix looks correct to me. Moving next_refresh_jiffies into struct damon_sysfs_kdamond gives each kdamond ownership of its refresh schedule, so refresh activity from one kdamond can no longer interfere with another. I also checked the initialization path: the new field is zero-initialized by kzalloc_obj(), and damon_sysfs_turn_damon_on() sets it to jiffies + refresh_ms before registering the callback via damon_call(). Reviewed-by: Kunwu Chan Thanks, Kunwu > > repeat_call_control->fn = damon_sysfs_repeat_call_fn; > repeat_call_control->data = kdamond; > -- > 2.47.3 > Sent using hkml (https://github.com/sjp38/hackermail)