From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932426AbbHKVpr (ORCPT ); Tue, 11 Aug 2015 17:45:47 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:35467 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752757AbbHKVpq (ORCPT ); Tue, 11 Aug 2015 17:45:46 -0400 Date: Tue, 11 Aug 2015 14:45:45 -0700 From: Greg Kroah-Hartman To: Ankit Jindal Cc: linux-kernel@vger.kernel.org, "Hans J. Koch" , Pranavkumar Sawargaonkar , Jitendra Kanitkar , Tushar Jagad Subject: Re: Unmapping of UIO logical memory causing a trace Message-ID: <20150811214545.GA1457@kroah.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23+102 (2ca89bed6448) (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 11, 2015 at 04:39:08PM +0530, Ankit Jindal wrote: > Hi, > > We have observed an issue where kmalloc of a small sized memory causes > an occasional trace when unmapping the mmaped memory via UIO framework > This trace is coming when kernel sees a negative value in > page->_mapcount. Trace is pasted at the end of the mail. > > After debugging this issue further, we realized following sequence > occurs when kmalloc is used to allocate small memory using slub > allocator: > 1. Frozen bit (msb) of the page from which memory has been allocated > is set (which is an union with _mapcount). > 2. If there are free objects in the the same page then this frozen bit > remains set even after kernel boots completely. > 3. When user space calls unmap of this memory, vma_unmap_single() > treats the _mapcount as a negative (as frozen bit is set), causing a > trace. > > We are not sure whether exposing kernel memory of size > less than PAGE_SIZE via UIO is a valid use case ? In case this is an invalid > use case then shouldn't the UIO framework restrict mapping of non > PAGE_SIZE aligned memory and size not in order of PAGE_SIZE. We've had a few discussions about this in the past, and one proposed patch which had to be reverted because it broke some working systems, so it's a messy thing. What UIO driver are you using that causes this behavior? thanks, greg k-h