From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out199-18.us.a.mail.aliyun.com (out199-18.us.a.mail.aliyun.com [47.90.199.18]) (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 D03357C for ; Thu, 18 Aug 2022 08:37:57 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R131e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045176;MF=liusong@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0VMZx3Mr_1660811864; Received: from 30.178.80.93(mailfrom:liusong@linux.alibaba.com fp:SMTPD_---0VMZx3Mr_1660811864) by smtp.aliyun-inc.com; Thu, 18 Aug 2022 16:37:46 +0800 Message-ID: Date: Thu, 18 Aug 2022 16:37:44 +0800 Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.0.3 Subject: Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within dma_pool_alloc To: Christoph Hellwig , Robin Murphy , akpm@linux-foundation.org Cc: Marek Szyprowski , "linux-arm-kernel@lists.infradead.org" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "iommu@lists.linux.dev" References: <1658125690-76930-1-git-send-email-liusong@linux.alibaba.com> <1dbe63ff-5575-745b-653a-a992ae53e1aa@samsung.com> <413d8666-7a82-efd7-6716-13658016ca10@arm.com> <20220817053628.GA28747@lst.de> From: Liu Song In-Reply-To: <20220817053628.GA28747@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit A helper function "use_dev_coherent_memory" is introduced here to >>>> determine whether the memory is allocated by "dma_alloc_from_dev_coherent". >>>> >>>> And use "get_dma_ops" to determine whether the memory is allocated by >>>> "dma_direct_alloc". > WTF? get_dma_ops is privat to the DMA API layer, and dmapool has no > business even using that. Even independent of this particular case, > consumers of an API never have any business looking at the implementation > of the API, that is the whole point of the abstraction. > >> It's not even that, the change here is just obviously broken, since it ends >> up entirely ignoring want_init_on_alloc() for devices using dma-direct. >> Sure, the memory backing a dma_page is zeroed *once* by its initial >> dma-coherent allocation, but who says we're not not reallocating pool >> entries from an existing dma_page? > And yes, in addition to that it also is completely broken. After reading everyone's comments, I found that fixing this patch will make the code look strange, so the benefits of the changes will be dispensable, so I also agree to discard this patch. Sorry for this trouble again. Thanks