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Wed, 9 Sep 2026 09:04:56 -0400 (EDT) Date: Wed, 9 Sep 2026 15:01:49 +0200 From: Greg KH To: SJ Park Cc: stable@vger.kernel.org, damon@lists.linux.dev, Andrew Morton Subject: Re: [PATCH 6.18.y] mm/damon/sysfs: read ops_id only once in damon_sysfs_apply_inputs() Message-ID: <2026090945-spookily-leotard-ecf3@gregkh> References: <2026090858-arousal-getting-ee35@gregkh> <20260909032830.119383-1-sj@kernel.org> Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260909032830.119383-1-sj@kernel.org> On Tue, Sep 08, 2026 at 08:28:30PM -0700, SJ Park wrote: > damon_sysfs_apply_inputs() reads ops_id twice. It could race with > ops_id_store(). As a result, the min_region_sz could wrongly be set up. > Read it once. > > The user impact is trivial. Sane users ain't update the parameter in > parallel. Even if it happens, the DAMON core layer handles the wrong > min_region_sz (!is_power_of_2()). Even if somehow the race ended up > making a min_region_sz that is different from the user's intention but > still valid, only monitoring itself runs differently than expected. No > critical consequences like kernel panic or memory corruption happen > > The issue was discovered [1] by Sashiko. > > Link: https://lore.kernel.org/20260715031002.108504-7-sj@kernel.org > Link: https://lore.kernel.org/20260703172417.95426-1-sj@kernel.org [1] > Fixes: 8d009da32f13 ("mm/damon/sysfs: set damon_ctx->min_sz_region only for paddr use case") > Signed-off-by: SJ Park > Cc: # 6.18.x > Signed-off-by: Andrew Morton > (cherry picked from commit 5adaaa28be8a79ddd7e103b171f9d6e14e7fc26e) > Signed-off-by: SJ Park > --- > mm/damon/sysfs.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c > index a6c3e83a3dad5..655e4de69bee8 100644 > --- a/mm/damon/sysfs.c > +++ b/mm/damon/sysfs.c > @@ -1432,14 +1432,16 @@ static inline bool damon_sysfs_kdamond_running( > static int damon_sysfs_apply_inputs(struct damon_ctx *ctx, > struct damon_sysfs_context *sys_ctx) > { > + enum damon_ops_id ops_id; > int err; > > - err = damon_select_ops(ctx, sys_ctx->ops_id); > + ops_id = READ_ONCE(sys_ctx->ops_id); > + err = damon_select_ops(ctx, ops_id); > if (err) > return err; > ctx->addr_unit = sys_ctx->addr_unit; > /* addr_unit is respected by only DAMON_OPS_PADDR */ > - if (sys_ctx->ops_id == DAMON_OPS_PADDR) > + if (ops_id == DAMON_OPS_PADDR) > ctx->min_sz_region = max( > DAMON_MIN_REGION / sys_ctx->addr_unit, 1); > err = damon_sysfs_set_attrs(ctx, sys_ctx->attrs); > -- > 2.47.3 > > Does not apply :(