From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta12.messagelabs.com (mail6.bemta12.messagelabs.com [216.82.250.247]) by kanga.kvack.org (Postfix) with ESMTP id 7382E6B0169 for ; Tue, 16 Aug 2011 10:27:43 -0400 (EDT) From: Arnd Bergmann Subject: Re: [PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings Date: Tue, 16 Aug 2011 16:26:25 +0200 References: <1313146711-1767-1-git-send-email-m.szyprowski@samsung.com> <201108161528.48954.arnd@arndb.de> <20110816135516.GC17310@n2100.arm.linux.org.uk> In-Reply-To: <20110816135516.GC17310@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201108161626.26130.arnd@arndb.de> Sender: owner-linux-mm@kvack.org List-ID: To: Russell King - ARM Linux Cc: Marek Szyprowski , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-media@vger.kernel.org, linux-mm@kvack.org, linaro-mm-sig@lists.linaro.org, Michal Nazarewicz , Kyungmin Park , Andrew Morton , KAMEZAWA Hiroyuki , Ankita Garg , Daniel Walker , Mel Gorman , Jesse Barker , Jonathan Corbet , Shariq Hasnain , Chunsang Jeong On Tuesday 16 August 2011, Russell King - ARM Linux wrote: > On Tue, Aug 16, 2011 at 03:28:48PM +0200, Arnd Bergmann wrote: > > Hmm, I don't remember the point about dynamically sizing the pool for > > ARMv6K, but that can well be an oversight on my part. I do remember the > > part about taking that memory pool from the CMA region as you say. > > If you're setting aside a pool of pages, then you have to dynamically > size it. I did mention during our discussion about this. > > The problem is that a pool of fixed size is two fold: you need it to be > sufficiently large that it can satisfy all allocations which come along > in atomic context. Yet, we don't want the pool to be too large because > then it prevents the memory being used for other purposes. > > Basically, the total number of pages in the pool can be a fixed size, > but as they are depleted through allocation, they need to be > re-populated from CMA to re-build the reserve for future atomic > allocations. If the pool becomes larger via frees, then obviously > we need to give pages back. Ok, thanks for the reminder. I must have completely missed this part of the discussion. When I briefly considered this problem, my own conclusion was that the number of atomic DMA allocations would always be very low because they tend to be short-lived (e.g. incoming network packets), so we could ignore this problem and just use a smaller reservation size. While this seems to be true in general (see "git grep -w -A3 dma_alloc_coherent | grep ATOMIC"), there is one very significant case that we cannot ignore, which is pci_alloc_consistent. This function is still called by hundreds of PCI drivers and always does dma_alloc_coherent(..., GFP_ATOMIC), even for long-lived allocations and those that are too large to be ignored. So at least for the case where we have PCI devices, I agree that we need to have the dynamic pool. Arnd -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org