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 C094C30F957 for ; Thu, 16 Apr 2026 04:06:04 +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=1776312364; cv=none; b=UUUwFR/C0oDsW+bDSd1UDYquYB1INHChTj1AD3j7QtKVfvaSjbIx7GogCC4Y14DamgDwKKquLbkiuB4k31tbc7Bz0anlIOaJLF0yISKLPY1fDJG6Wli0Kqa8PinHSv0VfHMN071BkgsS99ko8SQihqFl1ieKzJQG+WeCgjlslNY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776312364; c=relaxed/simple; bh=8wRN+H/SQYxjKdjXgNtCgwHJm/4TKR5Qc1q1r+kXOFw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=X9LJJkQBTzW/SXEyF2pkVwW8KqShaLyAl5qQUQj2wZbi/nOXdxUwbMxSQtrOzxMELX7j/1ayvMzLfQ14Ld2Y/op0MR46Gr6qya1SB/A1v5jyGaSMvbsu0euvf8t3Py9xUQZVUuPzpaC062QkEC/GrbrvetB9e0+SH7SxgbVbu5Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oHAVc/ox; 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="oHAVc/ox" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3F475C2BCAF; Thu, 16 Apr 2026 04:06:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776312364; bh=8wRN+H/SQYxjKdjXgNtCgwHJm/4TKR5Qc1q1r+kXOFw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=oHAVc/oxDyCTM6i12X7V7feHMaFO6X8J04heZK8FJrl+xKgX4wYzdDQwSKWhLgtQf WUIyw4QAnPv12raLddk6i00uyR4PrdA4MmP1J52QeyanaYXWma2GPF3Jrqp1xkyQfa gGX6FSn2kt7chdgarlXHV4+NjBsYUVhCQyJD4dZCteoDBG1LWKEHLiV5hXaHppOdGi VoChUHog0IkkRZayVG5x+QzvSx4qwSQJWxg+DLyB8t+ZByVWIUsm839SjEzqcnTvsU 5dAjYxU6n/rMofvh7HoJ4cuy9kQaOO3m3nbUO3y+QmL1R/a1OWpDFh/RxbFmhgI2o4 dpbkRwtNSvL8Q== From: SeongJae Park To: SeongJae Park Cc: sashiko-bot@kernel.org, damon@lists.linux.dev Subject: Re: [RFC PATCH v1.2 1/2] mm/damon/stat: add a parameter for reading kdamond pid Date: Wed, 15 Apr 2026 21:06:01 -0700 Message-ID: <20260416040602.88665-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260416033546.87941-1-sj@kernel.org> References: Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Wed, 15 Apr 2026 20:35:45 -0700 SeongJae Park wrote: > TL; DR: Sashiko found good point. I will fix it in the next revision. > > On Thu, 16 Apr 2026 00:59:35 +0000 sashiko-bot@kernel.org wrote: > > > Sashiko AI review found 1 potential issue(s): > > - [Medium] Stale `kdamond_pid` exposed to userspace if `damon_call()` fails during startup. > > -- > > > > commit 46eb0a4278ccd2dec190db28b78b0766b8865901 > > Author: SeongJae Park > > > > mm/damon/stat: add a parameter for reading kdamond pid > > > > This commit exposes the pid of the DAMON_STAT kdamond via a new read-only > > module parameter named kdamond_pid, standardizing usage with other DAMON > > modules. > > > > > diff --git a/mm/damon/stat.c b/mm/damon/stat.c > > > index 99ba346f9e325..4901e9a7c3398 100644 > > > --- a/mm/damon/stat.c > > > +++ b/mm/damon/stat.c > > > > [ ... ] > > > > > @@ -260,6 +269,14 @@ static int damon_stat_start(void) > > > damon_stat_context = NULL; > > > return err; > > > } > > > + kdamond_pid = damon_kdamond_pid(damon_stat_context); > > > + if (kdamond_pid < 0) { > > > + err = kdamond_pid; > > > + kdamond_pid = -1; > > > + damon_destroy_ctx(damon_stat_context); > > > + damon_stat_context = NULL; > > > + return err; > > > + } > > > > > > damon_stat_last_refresh_jiffies = jiffies; > > > call_control.data = damon_stat_context; > > > > If damon_call() fails right after this and returns an error, > > damon_stat_start() will pass that error back to its caller and the module > > will revert to a disabled state. > > > > Does this leave a stale kdamond_pid exposed to userspace? The newly added > > documentation states: "If DAMON_STAT is enabled, this becomes the PID of > > the worker thread. Else, -1." Since the initialization fails, the module > > remains disabled, but kdamond_pid isn't reset to -1 on this error path. > > Nice catch. I will do the kdamond_pid setup after damon_call() success, like > below. Wait... The same question was also raised [1] by Sashiko on the previous version. And I was wrongly saying it is not this patch's fault. That was completely wrong. And the kdamond can stop even after the damon_kdamond_pid() call, if the region_store_histogram allocation fails. In the case, kdamond_pid will again be stale. 'enabled' parameter value will also be stale in the case. The worst part of it is that in the case, DAMON_STAT cannot be restarted before the system is reboot. The bug is similar to the one [2] that Liew found and working on. I will hold this patch and fix the bug first. > Also, I will remove the damon_stat_context deallocation and unset when > damon_kdamond_pid() failure, because it will be deallocated when > damon_stat_start() is called again. This also means my reply [1] to Sashiko's question about the damon_stat_context leak on the previous version was wrong. I was saying it is indeed a bug, but it was not. [1] https://lore.kernel.org/20260414053742.90296-1-sj@kernel.org [2] https://lore.kernel.org/20260415235529.86329-1-sj@kernel.org Thanks, SJ [...]