From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH v2] dma-direct: do not allocate a single page from CMA area Date: Mon, 4 Feb 2019 09:23:07 +0100 Message-ID: <20190204082307.GA5916@lst.de> References: <20190115215140.1545-1-nicoleotsuka@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190115215140.1545-1-nicoleotsuka@gmail.com> Sender: linux-kernel-owner@vger.kernel.org To: Nicolin Chen Cc: hch@lst.de, m.szyprowski@samsung.com, robin.murphy@arm.com, vdumpa@nvidia.com, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org List-Id: iommu@lists.linux-foundation.org On Tue, Jan 15, 2019 at 01:51:40PM -0800, Nicolin Chen wrote: > The addresses within a single page are always contiguous, so it's > not so necessary to allocate one single page from CMA area. Since > the CMA area has a limited predefined size of space, it might run > out of space in some heavy use case, where there might be quite a > lot CMA pages being allocated for single pages. > > This patch tries to skip CMA allocations of single pages and lets > them go through normal page allocations unless the allocation has > a DMA_ATTR_FORCE_CONTIGUOUS attribute. This'd save some resources > in the CMA area for further more CMA allocations, and it can also > reduce CMA fragmentations resulted from trivial allocations. That DMA_ATTR_FORCE_CONTIGUOUS flag does not make sense. A single page allocation is per defintion always contigous. > again: > - /* CMA can be used only in the context which permits sleeping */ > - if (gfpflags_allow_blocking(gfp)) { > + /* > + * CMA can be used only in the context which permits sleeping. > + * Since addresses within one PAGE are always contiguous, skip > + * CMA allocation for a single page to save CMA reserved space > + * unless DMA_ATTR_FORCE_CONTIGUOUS is flagged. > + */ > + if (gfpflags_allow_blocking(gfp) && > + (count > 1 || attrs & DMA_ATTR_FORCE_CONTIGUOUS)) { And my other concern is that this skips allocating from the per-device pool, which drivers might rely on. To be honest I'm not sure there is much of a point in the per-device CMA pool vs the traditional per-device coherent pool, but I'd rather change that behavior in a clearly documented commit with intentions rather as a side effect from a random optimization.