From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 8DA4D158A1E; Tue, 9 Apr 2024 21:42:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712698969; cv=none; b=B/g0wp9WvgV+Od/BNwMG4GI7lPI4tHs2qnGayH2uQMVcBCJfFSAZkXKA8Sif3LYE8nNJSCQi4c0kjKW4JYQhfHMN8kXcN8FB/NdZhL5QyTpEdZDlX9VdrZ//tlpA5pymP9BCWPPIKRlxoM+kNDxNCb7AXw8BsTp2nls3ZaKKle8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712698969; c=relaxed/simple; bh=VnI1mV5conMscaAY6LWaUtz+ynM+a4jRDb2r1ctiVko=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=iil2HtdTAhX7w74xJYNazIAQAik0ZX86RJ7Wozax2ytUbpNR/vyF2H5sVAqWOXJONtcQyhrmw4JVdn6zjBgCwGGL2vX9mzzrupd5CS18JbawAZdXPFyhnz+IuUS4jb74LR+EyhytAVkhXMFttE8x67FzfRh/IIVZL1GEIaed50g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=ud318HAF; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ud318HAF" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=cAZm87PzI6fyQ1m2jU/2dvY7F4KiYWybRLHCj3v2RmA=; b=ud318HAFAMuxza1NMA5qxCWVYS Q3RQ7jjtcx3N5nPxhhPAR0r9nI5meQpupzsoLYg/sLklNXZGNvq4mJldoHmAZlB8EiPlc66C1enlq 5v9zE4iPBgno4IMn85s2hY3bY/kxn2nymlq59ieFhSg6I4plV2yvY3X40V7jN6+qA4t8AL8cjDmKV QL1wHhqVrLEbkmzXAyAszoqKaMDh+yZfRq646kXMiDKPVWpKg079mOaofT7ZiO68GJv1A7tl2Inw5 ry5F7XRa498awRyp/nbS9D2xQlZqxN9g3+IyGZhZ2iwmQQDGSZCm5jlUbR8JiU25qsEUpJwq5Ir6G 6HhHRSJw==; Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1ruJEb-00000002zqV-2c7l; Tue, 09 Apr 2024 21:42:29 +0000 Date: Tue, 9 Apr 2024 22:42:29 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, cgroups@vger.kernel.org, linux-sh@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Peter Xu , Ryan Roberts , Yin Fengwei , Yang Shi , Zi Yan , Jonathan Corbet , Hugh Dickins , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , Chris Zankel , Max Filippov , Muchun Song , Miaohe Lin , Naoya Horiguchi , Richard Chang Subject: Re: [PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again Message-ID: References: <20240409192301.907377-1-david@redhat.com> <20240409192301.907377-2-david@redhat.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240409192301.907377-2-david@redhat.com> On Tue, Apr 09, 2024 at 09:22:44PM +0200, David Hildenbrand wrote: > Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type() > pages") made it impossible to detect mapcount underflows by treating > any negative raw mapcount value as a mapcount of 0. Yes, but I don't think this is the right place to check for underflow. We should be checking for that on modification, not on read. I think it's more important for page_mapcount() to be fast than a debugging aid.