From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx173.postini.com [74.125.245.173]) by kanga.kvack.org (Postfix) with SMTP id 1668C6B0044 for ; Wed, 22 Aug 2012 23:49:08 -0400 (EDT) Date: Thu, 23 Aug 2012 12:49:35 +0900 From: Minchan Kim Subject: Re: [RFC 2/4] ARM: dma-mapping: IOMMU allocates pages from pool with GFP_ATOMIC Message-ID: <20120823034935.GC5369@bbox> References: <1345630830-9586-1-git-send-email-hdoyu@nvidia.com> <1345630830-9586-3-git-send-email-hdoyu@nvidia.com> <20120822.163648.3800987367886904.hdoyu@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120822.163648.3800987367886904.hdoyu@nvidia.com> Sender: owner-linux-mm@kvack.org List-ID: To: Hiroshi Doyu Cc: "pullip.cho@samsung.com" , "m.szyprowski@samsung.com" , "linux-arm-kernel@lists.infradead.org" , "linaro-mm-sig@lists.linaro.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "kyungmin.park@samsung.com" , "arnd@arndb.de" , "linux@arm.linux.org.uk" , "chunsang.jeong@linaro.org" , Krishna Reddy , "konrad.wilk@oracle.com" , "subashrp@gmail.com" On Wed, Aug 22, 2012 at 03:36:48PM +0200, Hiroshi Doyu wrote: > Hi, > > KyongHo Cho wrote @ Wed, 22 Aug 2012 14:47:00 +0200: > > > vzalloc() call in __iommu_alloc_buffer() also causes BUG() in atomic context. > > Right. > > I've been thinking that kzalloc() may be enough here, since > vzalloc() was introduced to avoid allocation failure for big chunk of > memory, but I think that it's unlikely that the number of page array > can be so big. So I propose to drop vzalloc() here, and just simply to > use kzalloc only as below(*1). > > For example, > > 1920(H) x 1080(W) x 4(bytes) ~= 8MiB > > For 8 MiB buffer, > 8(MiB) * 1024 = 8192(KiB) > 8192(KiB) / 4(KiB/page) = 2048 pages > sizeof(struct page *) = 4 bytes > 2048(pages) * 4(bytes/page) = 8192(bytes) = 8(KiB) > 8(KiB) / 4(KiB/page) = 2 pages > > If the above estimation is right(I hope;)), the necessary pages are > _at most_ 2 pages. If the system gets into the situation to fail to > allocate 2 contiguous pages, that's real the problem. I guess that > that kind of fragmentation problem would be solved with page migration > or something, especially nowadays devices are getting larger memories. In atomic context, VM have no choice except relying on kswapd so high order allocation can fail easily when memory fragementation is high. -- Kind regards, Minchan Kim -- 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/ . Don't email: email@kvack.org