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 1FC2F2F6160; Sat, 28 Mar 2026 17:48:53 +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=1774720134; cv=none; b=K66UFyTtNcIBAOA+p2boiZMKcmr7V2x+vxvdzLZlOdS+bTjQru+U9pCwW/vKENS9HarikBO3GJHyClyjflIFUQ4zCahWmZg+7TyWQ5hM4USppr2Li1tZGYAAbWytcXr18X9qMPIfmBqnZi3l7zzeZOu+AabXEHhvudGQWEqi4l4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774720134; c=relaxed/simple; bh=Sg6x97ANd8tIFA08/jJ7hgyZ6IOOblhEz9ehxsJhHE4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=LuFC5qCAiqbiWYOvZMYXFuFih10eEyuQLYVJvmY8/HsvOw8eGhWFdxYgl3oX8nkblFgb/72K6ECbEKqEEV5ltacQ9mLrMGrAcnnAn9Q5sxRw4SsW25VZBoMtiJrp0zOJRFb8fQlvFRdNlDaMtN5xcdZ2fiFDCDSPg7Y/PR0Kws0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CSnA9ZUk; 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="CSnA9ZUk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 51C1CC4CEF7; Sat, 28 Mar 2026 17:48:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774720133; bh=Sg6x97ANd8tIFA08/jJ7hgyZ6IOOblhEz9ehxsJhHE4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=CSnA9ZUkXUKgtI3r1nzkCgxnw3E+Ro571tGR8EixYrLYzvCdHdVOhSDusXh0Qwl8N o9peR2Fib9N7nsvBHy9XjVrzsQOBkbSoK0hPExVvWMR36AzLy/F7hxfKRItBe4om5w D0wr8BgX7DJcDwmENuxCtrZOdpdxw9deOOTykhvYA0o06mnA8Wj5m8lcNvWG5Gn7D2 aEsEYvhFcGmQvriiHQeW6oxRpXJTf3TTqO6icqJCdzSpz4SyJWvLJPdikz5WXHOczt zzsNO/T+hQBePKxLMs0xHrnvmLngBxNEZdp6XKwhERI6mnoWrCLMUbay7uvDEBQ5qx oNeXUHXD49PmQ== From: SeongJae Park To: SeongJae Park Cc: "Liam R. Howlett" , "# 5 . 19 . x" , Andrew Morton , David Hildenbrand , Jonathan Corbet , Lorenzo Stoakes , Michal Hocko , Mike Rapoport , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , damon@lists.linux.dev, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Liew Rui Yan Subject: Re: (sashiko review) [RFC PATCH 1/2] Docs/admin-guide/mm/damon/reclaim: warn commit_inputs vs param updates race Date: Sat, 28 Mar 2026 10:48:51 -0700 Message-ID: <20260328174852.53338-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260328173446.53021-1-sj@kernel.org> References: Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Sat, 28 Mar 2026 10:34:45 -0700 SeongJae Park wrote: > Forwarding Sashiko review for doing discussions via mails. > > # review url: https://sashiko.dev/#/patchset/20260328172415.49940-2-sj@kernel.org > # start of sashiko.dev inline review > commit 8b189801fc104c5ecc412454a3d8db6a3faf27d6 > Author: SeongJae Park > > Docs/admin-guide/mm/damon/reclaim: warn commit_inputs vs param updates race > > This commit adds documentation warning users not to write to module > parameters while commit_inputs is in progress. This warning is intended to > help users avoid a race condition that can cause undefined kernel behavior. > > Link: https://lore.kernel.org/20260319161620.189392-3-objecting@objecting.org > > > diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst > > --- a/Documentation/admin-guide/mm/damon/reclaim.rst > > +++ b/Documentation/admin-guide/mm/damon/reclaim.rst > > @@ -71,6 +71,10 @@ of parametrs except ``enabled`` again. Once the re-reading is done, this > > parameter is set as ``N``. If invalid parameters are found while the > > re-reading, DAMON_RECLAIM will be disabled. > > > > +Once ``Y`` is written to this parametr, the user must not write to any > > This isn't a bug, but there is a typo ("parametr") in this line. Good eye, I will fix this in the next revision. > > > +parameters until reading ``commit_inputs`` again returns ``N``. If users > > +violate this rule, the kernel can do undefined behaviors. > > Is it acceptable to document a race condition that leads to undefined > behavior instead of fixing it? > > Even if the module parameter interface is restricted to root, user space > shouldn't be able to trigger kernel undefined behavior (which could lead to > memory corruption or crashes) simply by writing to module parameters > concurrently. > > Should the underlying race condition in DAMON_RECLAIM be fixed with proper > synchronization rather than relying on userspace adherence to a documentation > warning? Good callout. In short, I agree for mainline kernel, but I'd argue this may be enough for stable kernels. That is, I was also thinking in a way similar to sashiko. But, from the work I realized the amount of changes for fixing those in stable-backporting-easy way is not small. Also, given the age of the broken commits (5.19!), I don't think this is urgent. And again arguably, as long as it is well documented, letting users break kernel when they want (e.g., sysrq-trigger) is ok...? Let me know if other humans have different opinion, though. But for future (mainline), surely this have to be fixed in the code. And Liew's patch [1] was initially started not for exactly this patch, but will fix this together. Cc-ing Liew. So, I'm suggesting to add this documentation for stable kernels, and continue helping Liew's patch for cleanly removing this awkward behavior in the mianline and future kernels. > > This isn't a bug, but the phrasing "can do undefined behaviors" is slightly > awkward. Rephrasing to "may exhibit undefined behavior" might be more natural. Good catch. I will update so in the next version. [1] https://lore.kernel.org/20260328084524.5451-1-aethernet65535@gmail.com Thanks, SJ [...]