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Date: Wed, 22 Jul 2015 13:32:52 +0200 From: Vlastimil Babka <vbabka@...e.cz> To: David Rientjes <rientjes@...gle.com> CC: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, cbe-oss-dev@...ts.ozlabs.org, kvm@...r.kernel.org, Mel Gorman <mgorman@...e.de>, Greg Thelen <gthelen@...gle.com>, "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>, Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, Joonsoo Kim <iamjoonsoo.kim@....com>, Naoya Horiguchi <n-horiguchi@...jp.nec.com>, Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, Arnd Bergmann <arnd@...db.de>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Gleb Natapov <gleb@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Cliff Whickman <cpw@....com>, Robin Holt <robinmholt@...il.com> Subject: Re: [PATCH] mm: rename and document alloc_pages_exact_node 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@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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