From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 324543515C9; Tue, 4 Aug 2026 15:33:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785857590; cv=none; b=uV6k8tH1jjdXkL9FjvPKP7lZafmFUt3rnZh7IkoyEK7yutzSbPsae5/UfO+hufSUgtjDlze6q7/AUIF2BQfNCFwj6THd1VjYKX56qif6yoGQTdFITVTiKmcHwyt2AUOcllVGfCntP6Xt8CTzaWGn/FXQaPBV3qvQJ3V9NhNDuuw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785857590; c=relaxed/simple; bh=oXqu5CkAgTmUeweinC3Djt5R3DdkUMWMFL1HcVjqEVw=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=u7oTRBldGclQ1uzBIU+gPubrr6Jp9LgU5zj3cwf9Gm5UPLWJIEeF2Dy7oczx/fDPJ959ZfBBxhSs1u6rc5qrwG/Qy5UfqKrk+5DrAtBDCq293DcKH75oV5FMPdU3+qT6MHuE+VGy5SaUAc3AqWwrFuVE9Qq+IuIZ2tcSRq6ZBoM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=R3oNVVsx; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="R3oNVVsx" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 05A651F000E9; Tue, 4 Aug 2026 15:33:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785857588; bh=+B2tyHlxtOIYFFAEVjuBTTR4kj9wHrzaGATG1AU1+pI=; h=Date:Subject:To:References:From:In-Reply-To; b=R3oNVVsxuCEWsgVHp9L06whtvAu65m31eI7YTUDVmBGTciHjhmbGlv6QUnaWentP6 iKLvzeiIIVg5Ghu1QbNkxlP7j4IS8pCBYrP2hqN5M3VwDO3VNlOGkYXUJZbP/QqIUY jDQEIQqJorCqz2ZmxeN9MyAQ4406Pc51zTU7Sf3uVL3FNPQ6V/gAqkzY9x567zrxhM bfstcUfhbLbTZldGnVZ+Xe+REaXQCRTAtHrfQNoDSivsMH8GjVZgEpiFVsUhWgRs6D Kj6Ww6zTa/zxFhnYHtMLGQpKCTNtF/CX24S8/PBlYm3B3hGjvCYaLeiw9n/12iBL7O avzTCd2u4JoOQ== Message-ID: <2a4423d6-7934-46b2-97da-a20c372b0d17@kernel.org> Date: Tue, 4 Aug 2026 17:33:02 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 1/3] mm/vmstat, mm/memcontrol: add _monotonic vmstat readers Content-Language: en-US To: Usama Arif , Andrew Morton , david@kernel.org, ljs@kernel.org, liam@infradead.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, kasong@tencent.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chrisl@kernel.org, nphamcs@gmail.com, baoquan.he@linux.dev, youngjun.park@lge.com, hannes@cmpxchg.org, roman.gushchin@linux.dev, muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, rientjes@google.com, kernel-team@meta.com References: <20260727162550.2032-1-usama.arif@linux.dev> <20260727162550.2032-2-usama.arif@linux.dev> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/27/26 18:23, Usama Arif wrote: > lruvec_page_state(), node_page_state(), and global_node_page_state() > all clamp negative reads to zero on CONFIG_SMP so that a transient > per-CPU delta skew presents as zero pages rather than > as a garbage unsigned value. This is the right behaviour for > non-monotonic page-count readers. > > It is however incorrect for callers that snapshot a monotonically- > incremented event counter and compute a delta from two samples. > Once the underlying signed long wraps past LONG_MAX, the clamped read > drops to zero while the previously-recorded snapshot still holds the > pre-wrap value; the unsigned subtraction then underflows into a > ~2^31 spurious delta for 32-bit architecture and corrupts the > caller's accumulator. > > Add non-clamping siblings that return the underlying state value > cast to unsigned long: > > global_node_page_state_monotonic() > node_page_state_monotonic() > lruvec_page_state_monotonic() > > With both samples read via the _monotonic variant, unsigned modular > subtraction stays correct across a signed-long wraparound as long > as the true growth between two samples fits in unsigned long > (< 2^32 on 32-bit, < 2^64 on 64-bit); the 32-bit bound is the > practically-reachable one that motivates this helper. > > The variants are only safe for monotonically-incremented counters. > Non-monotonic page-count readers must keep using the existing > clamped helpers so transient negative reads still present as zero. > > This is a prerequisite for a later patch which > replaces the producer-side anon_cost/file_cost accumulators with a > read-side accumulator in prepare_scan_control() that samples > monotonic per-LRU vmstat counters (PGROTATE_*, NR_VMSCAN_WRITE, > WORKINGSET_RESTORE_*) via lruvec_page_state_monotonic() and folds > their unsigned modular deltas into lruvec->cost[].count. > > Acked-by: Johannes Weiner > Acked-by: Shakeel Butt > Signed-off-by: Usama Arif Acked-by: Vlastimil Babka (SUSE)