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Fri, 14 Aug 2026 02:12:29 +0000 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.600.51.1.1\)) Subject: Re: [PATCH v3 1/2] memcg: acquire peaks_lock when reading memory.peak From: Muchun Song In-Reply-To: <20260814012924.2388613-2-ridong.chen@linux.dev> Date: Fri, 14 Aug 2026 10:12:10 +0800 Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton , Tejun Heo , David Finkel , =?utf-8?Q?Michal_Koutn=C3=BD?= , "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" , "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" , linux-kernel@vger.kernel.org, Tao Cui , Ridong Chen Content-Transfer-Encoding: quoted-printable Message-Id: <811C3A33-6B05-40D5-8448-19D5BE882A83@linux.dev> References: <20260814012924.2388613-1-ridong.chen@linux.dev> <20260814012924.2388613-2-ridong.chen@linux.dev> To: Ridong Chen X-Mailer: Apple Mail (2.3864.600.51.1.1) X-Rspamd-Queue-Id: 28F13140004 X-Stat-Signature: 3rh9t4hn3upacwoyfyhnp4bbcctnmeqp X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1786673550-325875 X-HE-Meta: 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 vHSkyNew ASJQP8AXFYbpttc7JBcMLRcBO6ldE469gMAwoE5JyJHlVrdScq7rzevdKRxNh/1FJLyUE/3w3vNonNR8R6zkYC5Anp9OTiG336P8fi2yajGaFUz6XCYyy0BR8Uc93meqC0cJdkhuxX1nzANq0nXgYMkQ5Jbk430ZRI0eEwNrAis91zbzGlgDHQGarMDO0tYODBD56STn/z2jwTQurXD+INowdnX2QGsLb0NU93y/TXQ/XHES10cGVEoFGN4DsV3nI9OHf3cye19nmaEdq4+0MGkIc32/l+VxnZV4Swda+lAgXdPA+NhA1S3Bu6tC/c5CwI2Qo+fM8rJ0gfJ0qAwXGt5eRZaSmUdRbu152IuRBDXs3aBDFs4HHk3x6+wWX2OkbuOcdQyfDEQVdgVwPeKtEyli68rA6GPQxdOXZD/U5XNeyh2GSnk3/YCXIprvGlO41QKU/ZD0sTj7piNlSQHZ3yL/VwOvor2FECItG Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Aug 14, 2026, at 09:29, Ridong Chen wrote: >=20 > From: Ridong Chen >=20 > Sashiko reported that a reader can transiently observe a lower peak > within a race window [1]. peak_show() returns > max(local_watermark, ofp->value), but peak_write() updates those two > under peaks_lock while the reader takes no lock. The interleaving is: >=20 > writer (reset on fd A) reader (fd B) > ---------------------- ------------- > usage =3D page_counter_read(pc) > WRITE_ONCE(local_watermark, usage) > // watermark lowered to usage > lw =3D = READ_ONCE(local_watermark) > // sees the lowered usage > val =3D READ_ONCE(ofp->value) > // B's value not updated yet > return max(lw, val) > // both low -> low peak > WRITE_ONCE(peer_ctx->value, usage) > // B updated, but too late >=20 > Fix it by acquiring peaks_lock when reading the peak, so the reader = sees > a consistent snapshot of local_watermark and the per-fd values. The = same > race applies to memory.swap.peak, which shares peaks_lock and the > peak_write() path, so take the lock there as well. >=20 > [1] = https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@linux.= dev?part=3D1 > Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write = handlers") > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Ridong Chen > Acked-by: Johannes Weiner > Acked-by: Shakeel Butt > --- > mm/memcontrol.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) >=20 > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 17da1f43b7d3..6dd8756870ff 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -4713,8 +4713,13 @@ static int peak_show(struct seq_file *sf, void = *v, struct page_counter *pc) > static int memory_peak_show(struct seq_file *sf, void *v) > { > struct mem_cgroup *memcg =3D mem_cgroup_from_css(seq_css(sf)); > + int ret; >=20 > - return peak_show(sf, v, &memcg->memory); > + spin_lock(&memcg->peaks_lock); Why not to use guard(spinlock)(&memcg->peaks_lock) to simplify the code. Muchun, Thanks. > + ret =3D peak_show(sf, v, &memcg->memory); > + spin_unlock(&memcg->peaks_lock); > + > + return ret; > } >=20 > static int peak_open(struct kernfs_open_file *of) > @@ -5858,8 +5863,13 @@ static u64 swap_current_read(struct = cgroup_subsys_state *css, > static int swap_peak_show(struct seq_file *sf, void *v) > { > struct mem_cgroup *memcg =3D mem_cgroup_from_css(seq_css(sf)); > + int ret; >=20 > - return peak_show(sf, v, &memcg->swap); > + spin_lock(&memcg->peaks_lock); > + ret =3D peak_show(sf, v, &memcg->swap); > + spin_unlock(&memcg->peaks_lock); > + > + return ret; > } >=20 > static ssize_t swap_peak_write(struct kernfs_open_file *of, char *buf, > --=20 > 2.34.1 >=20