Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751623AbaDQPkw (ORCPT ); Thu, 17 Apr 2014 11:40:52 -0400 Received: from mail-lb0-f177.google.com ([209.85.217.177]:41868 "EHLO mail-lb0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751076AbaDQPko (ORCPT ); Thu, 17 Apr 2014 11:40:44 -0400 MIME-Version: 1.0 In-Reply-To: <20140416124406.b6a3f8c9f6e7eb7328ebb5cb@linux-foundation.org> References: <1397567329-3771-1-git-send-email-akinobu.mita@gmail.com> <1397567329-3771-2-git-send-email-akinobu.mita@gmail.com> <20140416124406.b6a3f8c9f6e7eb7328ebb5cb@linux-foundation.org> Date: Fri, 18 Apr 2014 00:40:42 +0900 Message-ID: Subject: Re: [PATCH v3 1/5] x86: make dma_alloc_coherent() return zeroed memory if CMA is enabled From: Akinobu Mita To: Andrew Morton Cc: LKML , Marek Szyprowski , Konrad Rzeszutek Wilk , David Woodhouse , Don Dutile , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , x86@kernel.org, iommu@lists.linux-foundation.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2014-04-17 4:44 GMT+09:00 Andrew Morton : > On Tue, 15 Apr 2014 22:08:45 +0900 Akinobu Mita wrote: > >> Calling dma_alloc_coherent() with __GFP_ZERO must return zeroed memory. >> >> But when the contiguous memory allocator (CMA) is enabled on x86 and >> the memory region is allocated by dma_alloc_from_contiguous(), it >> doesn't return zeroed memory. Because dma_generic_alloc_coherent() >> forgot to fill the memory region with zero if it was allocated by >> dma_alloc_from_contiguous() >> >> Most implementations of dma_alloc_coherent() return zeroed memory >> regardless of whether __GFP_ZERO is specified. So this fixes it by >> unconditionally zeroing the allocated memory region. >> >> Alternatively, we could fix dma_alloc_from_contiguous() to return >> zeroed out memory and remove memset() from all caller of it. But we >> can't simply remove the memset on arm because __dma_clear_buffer() is >> used there for ensuring cache flushing and it is used in many places. >> Of course we can do redundant memset in dma_alloc_from_contiguous(), >> but I think this patch is less impact for fixing this problem. > > But this patch does a duplicated memset if the page was allocated by > alloc_pages_node()? You're right. Clearing __GFP_ZERO bit in gfp flags before allocating by alloc_pages_node() can fix this duplicated memset. > Would it not be better to pass the gfp_t to dma_alloc_from_contiguous() > and have it implement __GFP_ZERO? That will fix thsi inefficiency, > will be symmetrical with the other underlying allocators and should > permit the appropriate fixups in arm? Sounds good. If it also handles __GFP_WAIT, We can remove __GFP_WAIT check which is almost always required before calling dma_alloc_from_contiguous(). >> --- a/arch/x86/kernel/pci-dma.c >> +++ b/arch/x86/kernel/pci-dma.c >> @@ -97,7 +97,6 @@ void *dma_generic_alloc_coherent(struct device *dev, size_t size, >> >> dma_mask = dma_alloc_coherent_mask(dev, flag); >> >> - flag |= __GFP_ZERO; I'll soon prepare a follow-up patch to clear __GFP_ZERO like + flag &= ~__GFP_ZERO >> again: >> page = NULL; >> /* CMA can be used only in the context which permits sleeping */ >> @@ -120,7 +119,7 @@ again: >> >> return NULL; >> } >> - >> + memset(page_address(page), 0, size); >> *dma_addr = addr; >> return page_address(page); >> } >> -- >> 1.8.3.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/