Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6F33BC433FE for ; Tue, 21 Dec 2021 08:56:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235888AbhLUI43 (ORCPT ); Tue, 21 Dec 2021 03:56:29 -0500 Received: from verein.lst.de ([213.95.11.211]:45942 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233874AbhLUI42 (ORCPT ); Tue, 21 Dec 2021 03:56:28 -0500 Received: by verein.lst.de (Postfix, from userid 2407) id A0D3868AFE; Tue, 21 Dec 2021 09:56:23 +0100 (CET) Date: Tue, 21 Dec 2021 09:56:23 +0100 From: Christoph Hellwig To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Baoquan He , linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, hch@lst.de, cl@linux.com, John.p.donnelly@oracle.com, kexec@lists.infradead.org, stable@vger.kernel.org, Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka Subject: Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone Message-ID: <20211221085623.GA7733@lst.de> References: <20211213122712.23805-1-bhe@redhat.com> <20211213122712.23805-6-bhe@redhat.com> <20211213134319.GA997240@odroid> <20211214053253.GB2216@MiWiFi-R3L-srv> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 17, 2021 at 11:38:27AM +0000, Hyeonggon Yoo wrote: > My understanding is any buffer requested from kmalloc (without > GFP_DMA/DMA32) can be used by device driver because it allocates > continuous physical memory. It doesn't mean that buffer allocated > with kmalloc is free of addressing limitation. Yes. > > the addressing limitation comes from the capability of device, not > allocation size. if you allocate memory using alloc_pages() or kmalloc(), > the device has same limitation. and vmalloc can't be used for > devices because they have no MMU. vmalloc can be used as well, it just needs to be setup as a scatterlist and needs a little lover for DMA challenged platforms with the invalidate_kernel_vmap_range and flush_kernel_vmap_range helpers. > But we can map memory outside DMA zone into bounce buffer (which resides > in DMA zone) using DMA API. Yes, although in a few specific cases the bounce buffer could also come from somewhere else.