From nobody Sat Sep 26 21:12:25 2026 Received: from mta0.migadu.com (out-195.mta0.migadu.com [91.218.175.195]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4607A204C31 for ; Sun, 30 Aug 2026 00:21:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.195 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788049288; cv=none; b=uJYuiyfkPECKMtlURaEhjZbNyQlTQV9ej9RC+SXOBiXrFvvFEmEh48gn5Yhy0oLr8KXeDwya/aAF0u7mXHIfp4ewOvdLXfEjqPvKf2oJ3ZPPbLosMGi83xIAB37cJ6J+QJQn3vW/M7graieHoPicQ7zaI3uUO9JuwNHbvzME+X0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788049288; c=relaxed/simple; bh=QVlkSSSS5i8xu5m23D1SnKfFAtjaOMB8/90a5M4Lomc=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=upFxz+EM34vZyJVSKNinT0fTd/b/phenAuCR4WdChROapjG6dE1aeVBKM+QIH2nspplKCUtYRKx9pged22NiUR+OnqQkEPfAGi5oP69Ak6388dvHqxYdB3qdEqqM07M+SgQSuhfUl5PCoSbnmI+J3bK4gVVUuoVb7e4muCDlRvw= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=LY7mmaoF; arc=none smtp.client-ip=91.218.175.195 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="LY7mmaoF" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=QVlkSSSS5i8xu5m23D1SnKfFAtjaOMB8/90a5M4Lomc=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788049283; v=1; x=1788654083; b=LY7mmaoFxuUqir6SolTLvQ+wvq8vrtMjPYce7Zee0eOgPQXpj8wiy9VyV9C+3Kccn45pZhjw jiI3OEXMBfy1SNNWMhOkzo4y/6KiAgp+ifvwDKY9sQ/8kD1clnTnfS5rGW9K9odjlbrrCduLDZ8 Fx0eqTyxod/bxqrkadPTOJNU= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 5c14cddd30220827; Sun, 30 Aug 2026 00:21:22 +0000 X-Mizu-Trace-ID: 5c14cddd30220827 X-Migadu-Flow: FLOW_OUT From: Ridong Chen To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton Cc: Muchun Song , Tejun Heo , =?UTF-8?q?Michal=20Koutn=C3=BD?= , David Finkel , cgroups@vger.kernel.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-kernel@vger.kernel.org, Tao Cui , Ridong Chen , Ridong Chen Subject: [PATCH v4 RESEND 1/2] memcg: acquire peaks_lock when reading memory.peak Date: Sun, 30 Aug 2026 08:20:43 +0800 Message-Id: <20260830002044.1938621-2-ridong.chen@linux.dev> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260830002044.1938621-1-ridong.chen@linux.dev> References: <20260830002044.1938621-1-ridong.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Ridong Chen 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: 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 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. [1] https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@lin= ux.dev?part=3D1 Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers") Assisted-by: Claude:claude-opus-4-8 Acked-by: Johannes Weiner Acked-by: Shakeel Butt Reviewed-by: Muchun Song Signed-off-by: Ridong Chen --- mm/memcontrol.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 095050d4296a..b84f448fe984 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4719,6 +4719,7 @@ static int memory_peak_show(struct seq_file *sf, void= *v) { struct mem_cgroup *memcg =3D mem_cgroup_from_css(seq_css(sf)); =20 + guard(spinlock)(&memcg->peaks_lock); return peak_show(sf, v, &memcg->memory); } =20 @@ -5909,6 +5910,7 @@ static int swap_peak_show(struct seq_file *sf, void *= v) { struct mem_cgroup *memcg =3D mem_cgroup_from_css(seq_css(sf)); =20 + guard(spinlock)(&memcg->peaks_lock); return peak_show(sf, v, &memcg->swap); } =20 --=20 2.34.1 From nobody Sat Sep 26 21:12:25 2026 Received: from mta0.migadu.com (out-202.mta0.migadu.com [91.218.175.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2921318FC80 for ; Sun, 30 Aug 2026 00:21:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.202 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788049293; cv=none; b=qcIbDYnp2rWH8zOG/jD7ryciQ7esrLxloCdq0gf1aTJnqzMdia7zwoQTls33w9cGCUg57C1AlyQOJifXzcsC9EqZ4Hv1awPiTshj22Y8eU5zM3v4vlZFx3gvMO+NV9iVj4VaXwX0H0d8nlmPWp6OYX29IX8yjYl0+h1wgcVDB24= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788049293; c=relaxed/simple; bh=HW0I75C/5UBwwd+jukPw0ARJed7HblTEcNCAI5VbTdI=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=YWZZGtj1fxs5XKlzAQRTCVjGlu0xkVvAn21DOeNVDRG3NoHUw8LncQbdJic8Fvmt3E6iz9ktruag4zzMWFo5ueHhqgOpFwefL9sN0LhjQP+xIwx3u91eYjVDOYfm+RDmrX188JNMdYSm0X1FoXmFIuqoEr3M7Osh3YZB7y88M9M= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=cv5J/t3a; arc=none smtp.client-ip=91.218.175.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="cv5J/t3a" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=HW0I75C/5UBwwd+jukPw0ARJed7HblTEcNCAI5VbTdI=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788049289; v=1; x=1788654089; b=cv5J/t3aEBhM4DKLAaOZG4aDJjyvdVlwaVEnSmjLfQx0ZFmRa45LBUDqVbjlbP0DwkvrP81S FeQSBC6pO23xN4OYtcPP1Xggo7ZHjbu6LvSoMZubHavDI2ci1/0xGhb/KEdjtYW8Nu61ERUtFDr Bu+zv/3Yn6pU/bvtFpJ6H90w= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 485d79aea16e38b9; Sun, 30 Aug 2026 00:21:29 +0000 X-Mizu-Trace-ID: 485d79aea16e38b9 X-Migadu-Flow: FLOW_OUT From: Ridong Chen To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton Cc: Muchun Song , Tejun Heo , =?UTF-8?q?Michal=20Koutn=C3=BD?= , David Finkel , cgroups@vger.kernel.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-kernel@vger.kernel.org, Tao Cui , Ridong Chen , Ridong Chen Subject: [PATCH v4 RESEND 2/2] mm, memcg: fix memory.peak reset clobbering other fds' watermark Date: Sun, 30 Aug 2026 08:20:44 +0800 Message-Id: <20260830002044.1938621-3-ridong.chen@linux.dev> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260830002044.1938621-1-ridong.chen@linux.dev> References: <20260830002044.1938621-1-ridong.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Ridong Chen Writing to memory.peak resets the peak for that fd only. Each fd is a watcher and reads back max(its own value, the shared local_watermark). peak_write() resets by lowering local_watermark to the current usage. To keep the other watchers' peaks it then walks the watcher list, but it stores the current usage into them instead of the old watermark. So once usage has dropped from a peak, a reset on one fd wrongly drags every other fd's peak down too, even fds that never reset. Reproduced on 7.2.0-rc5-next under QEMU, two fds A and B on one cgroup: B sees the peak (410624 KB), usage drops, then A resets -- and B's peak collapses to 1060 KB although B never reset. With this patch B keeps reading 410624 KB. Fix: save the old watermark before lowering it and use that to floor the other watchers, so a reset only affects the fd that issued it. Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers") Closes: https://sashiko.dev/#/patchset/20260807090000.1532495-1-ridong.chen= @linux.dev Assisted-by: Claude:claude-opus-4-8 Acked-by: Tao Cui Acked-by: Johannes Weiner Acked-by: Shakeel Butt Signed-off-by: Ridong Chen --- mm/memcontrol.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index b84f448fe984..4c02d7de460c 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4749,7 +4749,7 @@ static ssize_t peak_write(struct kernfs_open_file *of= , char *buf, size_t nbytes, loff_t off, struct page_counter *pc, struct list_head *watchers) { - unsigned long usage; + unsigned long usage, old_watermark; struct cgroup_of_peak *peer_ctx; struct mem_cgroup *memcg =3D mem_cgroup_from_css(of_css(of)); struct cgroup_of_peak *ofp =3D of_peak(of); @@ -4757,11 +4757,12 @@ static ssize_t peak_write(struct kernfs_open_file *= of, char *buf, size_t nbytes, spin_lock(&memcg->peaks_lock); =20 usage =3D page_counter_read(pc); + old_watermark =3D READ_ONCE(pc->local_watermark); WRITE_ONCE(pc->local_watermark, usage); =20 list_for_each_entry(peer_ctx, watchers, list) - if (usage > peer_ctx->value) - WRITE_ONCE(peer_ctx->value, usage); + if (peer_ctx !=3D ofp && old_watermark > peer_ctx->value) + WRITE_ONCE(peer_ctx->value, old_watermark); =20 /* initial write, register watcher */ if (ofp->value =3D=3D OFP_PEAK_UNSET) --=20 2.34.1