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Wed, 31 Dec 2025 17:12:09 -0800 (PST) MIME-Version: 1.0 References: <20251231153216.82343-1-sj@kernel.org> In-Reply-To: <20251231153216.82343-1-sj@kernel.org> From: JaeJoon Jung Date: Thu, 1 Jan 2026 10:11:58 +0900 X-Gm-Features: AQt7F2qnT-PD4FKqqWfJZ823JyVjjLnvf35NzBoZo0u3yNRx2gXMQAYKNQHa-uY Message-ID: Subject: Re: [RFC PATCH v1] mm: improve call_controls_lock To: SeongJae Park Cc: Asier Gutierrez , akpm@linux-foundation.org, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, wangkefeng.wang@huawei.com, artem.kuzin@huawei.com, stepanov.anatoly@huawei.com Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 9A4A9C0003 X-Stat-Signature: wm3d8ttwtio35q1i6zjnxxkb1t5f5rk1 X-Rspam-User: X-HE-Tag: 1767229930-853650 X-HE-Meta: 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 LOurywhJ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 1 Jan 2026 at 00:32, SeongJae Park wrote: > > On Wed, 31 Dec 2025 15:10:12 +0900 JaeJoon Jung wrote: > > > On Wed, 31 Dec 2025 at 13:59, SeongJae Park wrote: > > > > > > On Wed, 31 Dec 2025 11:15:00 +0900 JaeJoon Jung wrote: > > > > > > > On Tue, 30 Dec 2025 at 00:23, SeongJae Park wrote: > > > > > > > > > > Hello Asier, > > > > > > > > > > > > > > > Thank you for sending this patch! > > > > > > > > > > On Mon, 29 Dec 2025 14:55:32 +0000 Asier Gutierrez wrote: > > > > > > > > > > > This is a minor patch set for a call_controls_lock synchronization improvement. > > > > > > > > > > Please break description lines to not exceed 75 characters per line. > > > > > > > > > > > > > > > > > Spinlocks are faster than mutexes, even when the mutex takes the fast > > > > > > path. Hence, this patch replaces the mutex call_controls_lock with a spinlock. > > > > > > > > > > But call_controls_lock is not being used on performance critical part. > > > > > Actually, most of DAMON code is not performance critical. I really appreciate > > > > > your patch, but I have to say I don't think this change is really needed now. > > > > > Please let me know if I'm missing something. > > > > > > > > Paradoxically, when it comes to locking, spin_lock is better than > > > > mutex_lock > > > > because "most of DAMON code is not performance critical." > > > > > > > > DAMON code only accesses the ctx belonging to kdamond itself. For > > > > example: > > > > kdamond.0 --> ctx.0 > > > > kdamond.1 --> ctx.1 > > > > kdamond.2 --> ctx.2 > > > > kdamond.# --> ctx.# > > > > > > > > There is no cross-approach as shown below: > > > > kdamond.0 --> ctx.1 > > > > kdamond.1 --> ctx.2 > > > > kdamond.2 --> ctx.0 > > > > > > > > Only the data belonging to kdamond needs to be resolved for concurrent access. > > > > most DAMON code needs to lock/unlock briefly when add/del linked > > > > lists, > > > > so spin_lock is effective. > > > > > > I don't disagree this. Both spinlock and mutex effectively work for DAMON's > > > locking usages. > > > > > > > If you handle it with a mutex, it becomes > > > > more > > > > complicated because the rescheduling occurs as a context switch occurs > > > > inside the kernel. > > > > > > Can you please elaborate what kind of complexities you are saying about? > > > Adding some examples would be nice. > > > > > > > Moreover, since the call_controls_lock that is > > > > currently > > > > being raised as a problem only occurs in two places, the kdamon_call() > > > > loop > > > > and the damon_call() function, it is effective to handle it with a > > > > spin_lock > > > > as shown below. > > > > > > > > @@ -1502,14 +1501,15 @@ int damon_call(struct damon_ctx *ctx, struct > > > > damon_call_control *control) > > > > control->canceled = false; > > > > INIT_LIST_HEAD(&control->list); > > > > > > > > - mutex_lock(&ctx->call_controls_lock); > > > > + spin_lock(&ctx->call_controls_lock); > > > > + /* damon_is_running */ > > > > if (ctx->kdamond) { > > > > list_add_tail(&control->list, &ctx->call_controls); > > > > } else { > > > > - mutex_unlock(&ctx->call_controls_lock); > > > > + spin_unlock(&ctx->call_controls_lock); > > > > return -EINVAL; > > > > } > > > > - mutex_unlock(&ctx->call_controls_lock); > > > > + spin_unlock(&ctx->call_controls_lock); > > > > > > > > if (control->repeat) > > > > return 0; > > > > > > Are you saying the above diff can fix the damon_call() use-after-free bug [1]? > > > Can you please elaborate why you think so? > > > > > > [1] https://lore.kernel.org/20251231012315.75835-1-sj@kernel.org > > > > > > > The above code works fine with spin_lock. However, when booting the kernel, > > the spin_lock call trace from damon_call() is output as follows: > > If you have any experience with the following, please share it. > > Can you please reply to my questions above, first? I have answered your above question. And, since call_controls_lock has a short waiting time, I think it would be a good idea to consider spin_lock. Thanks, JaeJoon > > > Thanks, > SJ > > [...]