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b=IBgdRi4WLL5c6IT9JCcsrC3Etn/VrD/bX+BZIoS+CjgstzmHcjujz5l97WL5TfpU7 fAt65hh/gBSEh7fM/KYlHa/u1aaxXTfsEBHQyH1wme0LH2ljNatZyN30ZmxATx78gL JZfniWKCAfrN7cvGOxmeF2Gf7X0lcDP47sjIujCXJgqhIqKWa6IPJ/tv+yL55ustIn nt09wisv7/gsy7cps6+JtqYwox32NOYU1QqKVkgaqXZ2HIXmgdWvpaao3ihRDcorQh /gByy1jglZnEIABPB9WGbpWi94BjrBQq/Y7DI808G3cOWeXO9XhkAwVbIX58pl48Vu cEpM+0qu4YhpA== From: SeongJae Park To: SeongJae Park Cc: Andrew Morton , Josh Law , "# 6 . 17 . x" , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v3 3/3] mm/damon/sysfs: check contexts->nr in repeat_call_fn Date: Sat, 21 Mar 2026 13:10:28 -0700 Message-ID: <20260321201029.95644-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260321200522.95395-1-sj@kernel.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 66737140007 X-Stat-Signature: yhur5nigus8363s6qr8sw6tz4bg483yn X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1774123838-852121 X-HE-Meta: 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 uVoFEPFd gG9Y/iyK52Kvan9We3P8nO9VCXpq/qnLZdn9JbXF/DGm3J10xC/0cGhQDQcvOo2KrAVyfV8TqXiAHgla3pjxfW8Y78WSZ1IjaN2s2JVeJ230r5cF4SinjmJfzPUaPwLgsovSBwOlTMxeMkgngL/NBwHrF5GPdcGY0NEfeEkqh1+ZRL/COAlVpiyXjLTFeoHJp8dNmK2uqofFbqhwgSeUbpM0qkE6zQ+PICtyHpMYsEF5rzHavLsS8QRAY5Egb6xV2Q7trqOSohKxeD3L3nJoN2E7AEx8++XGDDkhuTs7Iml41g01p+8hePzw25TFHqnvQckHIoV7JDQHEKoG+lmL38lcEAKwc0TbVAT8739eUr1Q5nvs/cZ0aAhLwkLBFEr8qbs2PPaWmUSzEntcOvkk4HvzmSUFAyX+l84kDpVBxFb7VjlMc+711zKmA3fNAl/4RgAZSANLswN0yVibw43Aw5xQxnB9bbbaScw9Z Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: TL; DR: no blocker for this patch is found. On Sat, 21 Mar 2026 13:05:22 -0700 SeongJae Park wrote: > Subject: (Sashiko) Re: [PATCH v3 3/3] mm/damon/sysfs: check contexts->nr in repeat_call_fn > > Forwarding Sashiko review for doing discussions via mails. > > # review url: https://sashiko.dev/#/patchset/20260321175427.86000-4-sj@kernel.org > # start of sashiko.dev inline review > commit 349107c02ee8a122119ac77a51451bb3d9311a34 > Author: Josh Law > > mm/damon/sysfs: check contexts->nr in repeat_call_fn > > damon_sysfs_repeat_call_fn() calls damon_sysfs_upd_tuned_intervals(), > damon_sysfs_upd_schemes_stats(), and > damon_sysfs_upd_schemes_effective_quotas() without checking > contexts->nr. If nr_contexts is set to 0 via sysfs while DAMON is > running, these functions dereference contexts_arr[0] and cause a NULL > pointer dereference. Add the missing check. > > Link: https://patch.msgid.link/20260320163559.178101-3-objecting@objecting.org > > > diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c > > index ddc30586c0e6..6a44a2f3d8fc 100644 > > --- a/mm/damon/sysfs.c > > +++ b/mm/damon/sysfs.c > > @@ -1620,9 +1620,12 @@ static int damon_sysfs_repeat_call_fn(void *data) > > > > if (!mutex_trylock(&damon_sysfs_lock)) > > return 0; > > + if (sysfs_kdamond->contexts->nr != 1) > > + goto out; > > damon_sysfs_upd_tuned_intervals(sysfs_kdamond); > > Does this code cause starvation or incorrect statistics when multiple > kdamonds are active? > > It appears that damon_sysfs_next_update_jiffies is declared as a static > global variable and used by this function to throttle updates: > > static unsigned long damon_sysfs_next_update_jiffies; > > static int damon_sysfs_repeat_call_fn(void *data) > { > ... > if (time_before(jiffies, damon_sysfs_next_update_jiffies)) > return 0; > damon_sysfs_next_update_jiffies = jiffies + > msecs_to_jiffies(sysfs_kdamond->refresh_ms); > ... > } > > If multiple kdamonds run concurrently, wouldn't they all share and > overwrite this single global timestamp? > > If one kdamond sets the timestamp to a future value, another kdamond's > thread might incorrectly evaluate time_before() to true and skip its > update, potentially leading to starvation and incorrect sysfs > statistics for all but the winning kdamond. I answered this in the previous version of this patch. In short, good finding but not a blocker of this patch, I will work on it. > > > damon_sysfs_upd_schemes_stats(sysfs_kdamond); > > damon_sysfs_upd_schemes_effective_quotas(sysfs_kdamond); > > +out: > > mutex_unlock(&damon_sysfs_lock); > > return 0; > > } > > Could there also be a memory leak of repeat_call_control in > damon_sysfs_turn_damon_on() if the target kdamond exits early? > > In damon_sysfs_turn_damon_on(), repeat_call_control is allocated: > > repeat_call_control = kmalloc_obj(*repeat_call_control); > > If the kdamond thread encounters an error or has no work, it can exit > and set ctx->kdamond = NULL before damon_call() is fully executed. > > In this race window, if damon_call() detects that the context is > inactive it calls damon_call_handle_inactive_ctx(). This removes the > control from the list and returns -EINVAL, but it does not free the > memory. > > Because damon_sysfs_turn_damon_on() ignores the return value of > damon_call(), does this result in a permanent leak of the memory > allocated for repeat_call_control? I answered this in the previous version of this patch. In short, good finding but not a blocker of this patch, I will work on it. > > > # end of sashiko.dev inline review > # review url: https://sashiko.dev/#/patchset/20260321175427.86000-4-sj@kernel.org Thanks, SJ [...]