From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755527AbbJGTdy (ORCPT ); Wed, 7 Oct 2015 15:33:54 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:32831 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752179AbbJGTdw (ORCPT ); Wed, 7 Oct 2015 15:33:52 -0400 Date: Wed, 7 Oct 2015 12:33:51 -0700 From: Andrew Morton To: Robin Murphy Cc: Russell King - ARM Linux , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , "arnd@arndb.de" , "linux-mm@kvack.org" , "sakari.ailus@iki.fi" , "sumit.semwal@linaro.org" , "linux-arm-kernel@lists.infradead.org" , "m.szyprowski@samsung.com" Subject: Re: [PATCH 4/4] dma-debug: Allow poisoning nonzero allocations Message-Id: <20151007123351.763b98a8fe82a86ce7fdaf0c@linux-foundation.org> In-Reply-To: <56156FAF.9020002@arm.com> References: <0405c6131def5aa179ff4ba5d4201ebde89cede3.1443178314.git.robin.murphy@arm.com> <20150925124447.GO21513@n2100.arm.linux.org.uk> <560585EB.3060908@arm.com> <20150929142727.e95a2d2ebff65dda86315248@linux-foundation.org> <56156FAF.9020002@arm.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 7 Oct 2015 20:17:03 +0100 Robin Murphy wrote: > > It might be helpful to provide a runtime knob as well - having to > > rebuild&reinstall just to enable/disable this feature is a bit painful. > > Good point - there's always the global DMA debug disable knob, but this > particular feature probably does warrant finer-grained control to be > really practical. Having thought about it some more, it's also probably > wrong that this doesn't respect the dma_debug_driver filter, given that > it is actually invasive; in fixing that, how about if it also *only* > applied when a specific driver is filtered? Then there would be no > problematic "break anything and everything" mode, and the existing > debugfs controls should suffice. Yes, this should respect the driver filtering. On reflection... The patch poisons dma buffers if CONFIG_DMA_API_DEBUG and if __GFP_ZERO wasn't explicitly used. I'm rather surprised that the dma-debug code didn't do this from day one. I'd be inclined to enable this buffer-poisoning by default. Do you have a feeling for how much overhead that will add? Presumably not much, if __GFP_ZERO is acceptable. Also, how about we remove CONFIG_DMA_API_DEBUG_POISON and switch to a debugfs knob? btw, the documentation could do with a bit of a tune-up. The comments in dma-debug.c regarding driver filtering are non-existent. Documentation/kernel-parameters.txt says "The filter can be disabled or changed to another driver later using sysfs" but Documentation/DMA-API.txt talks about debugfs.