Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934241AbbGVLdF (ORCPT ); Wed, 22 Jul 2015 07:33:05 -0400 Received: from cantor2.suse.de ([195.135.220.15]:39951 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933837AbbGVLdD (ORCPT ); Wed, 22 Jul 2015 07:33:03 -0400 Message-ID: <55AF7F64.1040602@suse.cz> Date: Wed, 22 Jul 2015 13:32:52 +0200 From: Vlastimil Babka User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: David Rientjes CC: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, cbe-oss-dev@lists.ozlabs.org, kvm@vger.kernel.org, Mel Gorman , Greg Thelen , "Aneesh Kumar K.V" , Christoph Lameter , Pekka Enberg , Joonsoo Kim , Naoya Horiguchi , Tony Luck , Fenghua Yu , Arnd Bergmann , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Gleb Natapov , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Cliff Whickman , Robin Holt Subject: Re: [PATCH] mm: rename and document alloc_pages_exact_node References: <1437486951-19898-1-git-send-email-vbabka@suse.cz> In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3101 Lines: 64 On 07/21/2015 11:31 PM, David Rientjes wrote: > On Tue, 21 Jul 2015, Vlastimil Babka wrote: > >> The function alloc_pages_exact_node() was introduced in 6484eb3e2a81 ("page >> allocator: do not check NUMA node ID when the caller knows the node is valid") >> as an optimized variant of alloc_pages_node(), that doesn't allow the node id >> to be -1. Unfortunately the name of the function can easily suggest that the >> allocation is restricted to the given node. In truth, the node is only >> preferred, unless __GFP_THISNODE is among the gfp flags. >> >> The misleading name has lead to mistakes in the past, see 5265047ac301 ("mm, >> thp: really limit transparent hugepage allocation to local node") and >> b360edb43f8e ("mm, mempolicy: migrate_to_node should only migrate to node"). >> >> To prevent further mistakes, this patch renames the function to >> alloc_pages_prefer_node() and documents it together with alloc_pages_node(). >> > > alloc_pages_exact_node(), as you said, connotates that the allocation will > take place on that node or will fail. So why not go beyond this patch and > actually make alloc_pages_exact_node() set __GFP_THISNODE and then call > into a new alloc_pages_prefer_node(), which would be the current > alloc_pages_exact_node() implementation, and then fix up the callers? OK, but then we have alloc_pages_node(), alloc_pages_prefer_node() and alloc_pages_exact_node(). Isn't that a bit too much? The first two differ only in tiny bit: static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask, unsigned int order) { /* Unknown node is current node */ if (nid < 0) nid = numa_node_id(); return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask)); } static inline struct page *alloc_pages_prefer_node(int nid, gfp_t gfp_mask, unsigned int order) { VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES || !node_online(nid)); return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask)); } So _prefer_node is just a tiny optimization over the other one. It should be maybe called __alloc_pages_node() then? This would perhaps discourage users outside of mm/arch code (where it may matter). The savings of a skipped branch is likely dubious anyway... It would be also nice if alloc_pages_node() could use __alloc_pages_node() internally, but I'm not sure if all callers are safe wrt the VM_BUG_ON(!node_online(nid)) part. So when the alloc_pages_prefer_node is diminished as __alloc_pages_node or outright removed, then maybe alloc_pages_exact_node() which adds __GFP_THISNODE on its own, might be a useful wrapper. But I agree with Christoph it's a duplication of the gfp_flags functionality and I don't think there would be many users left anyway. -- 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/