From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 46074199230; Thu, 1 Jan 2026 02:00:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767232836; cv=none; b=MPalxMZM75t2mSrbSvEoSOFhHgySFWxv0fYMX0oeJO++M0DFBBazVjB/KThXzbZlbu68Lr+MVp5wl3Mnn+BNcGQXAJnyYd8QZZQdn/OrD6v/fcJW5lQsb9R1iXDO2OzZEo9h7kIeawx/MyHs4iKNpWLyjcqsTZnvInpL4XM7hDs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767232836; c=relaxed/simple; bh=SfUd+VIYbA5tyDlhKz1PyHaZjHMxYtaNAXFMWqOAQws=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=gXTv9zFxt7OKXlQ5A1Q1jYubiQa83vxZYt7sW0SuweTzoPhgqHem+7LtzpxMyxBDS5bbfe3f7LL8mxwOP1qh9iWREQPAoj9j01qrxBRqZFYX8hFDcjBgSleX2itI2B322de1DjCoXaHq6zDDfOpcWMmFIS95vc3RnHepMVxxk7o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IyEuIJMM; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IyEuIJMM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9E0A4C113D0; Thu, 1 Jan 2026 02:00:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767232835; bh=SfUd+VIYbA5tyDlhKz1PyHaZjHMxYtaNAXFMWqOAQws=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=IyEuIJMM5hM3OgqjM6myvFfoDMfB+64kXLBnhjT6N0VxJijiH5bYzArVhLmcKxOgL UfqyCYpeTqi/EMwNXZgwZd2rfOgWiKSSzFIWWdpWrnK62VmRLCkY3FSexzkEE8mZip qn033Qg7cmiVldfoGKW6cBWV0qCQpmaiJUQ+nDi2Htxvoo3HJ3zKF/VRhlk2cJc9TR uy3yGLrh1iDtpf4zWBJJNIpmSW0pgCOpjiVtVdbJYwUnG2hkzOnICEggYdMVfxHgVs 93mT3FhFD0xMBrtJSv22bM0g7aVgnQKWYl//I1I4YgIHShS3YVzssBK4Kmc7hlFXz5 n7o0wOj1RE0vQ== From: SeongJae Park To: JaeJoon Jung Cc: SeongJae Park , 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 Subject: Re: [RFC PATCH v1] mm: improve call_controls_lock Date: Wed, 31 Dec 2025 18:00:27 -0800 Message-ID: <20260101020028.88096-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, 1 Jan 2026 10:11:58 +0900 JaeJoon Jung wrote: > 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. Are you saying your reply [1] that posted today? Unfortunately I was unable to get all answers to my questions from it, so I asked your more explanation as a reply to that. > And, since call_controls_lock has a > short waiting time, I think it would be a good idea to consider spin_lock. This sounds like you are only repeating what you told so far, without additional explanation. Hopefully the additional explanation can be made on the thread [1]. Please keep replying there. [1] https://lore.kernel.org/CAHOvCC65azs4BU2fyP-kxvFWB3ZPCfyZ7KCO8N1sc0jtTENmNw@mail.gmail.com Thanks, SJ [...]