From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx125.postini.com [74.125.245.125]) by kanga.kvack.org (Postfix) with SMTP id 7D3C46B004D for ; Thu, 2 Aug 2012 06:32:51 -0400 (EDT) Date: Thu, 2 Aug 2012 03:32:44 -0700 From: Josh Triplett Subject: Re: [RFC 1/4] hashtable: introduce a small and naive hashtable Message-ID: <20120802103244.GA23318@leaf> References: <20120731182330.GD21292@google.com> <50197348.9010101@gmail.com> <20120801182112.GC15477@google.com> <50197460.8010906@gmail.com> <20120801182749.GD15477@google.com> <50197E4A.7020408@gmail.com> <20120801202432.GE15477@google.com> <5019B0B4.1090102@gmail.com> <20120801224556.GF15477@google.com> <501A4FC1.8040907@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <501A4FC1.8040907@gmail.com> Sender: owner-linux-mm@kvack.org List-ID: To: Sasha Levin Cc: Tejun Heo , torvalds@linux-foundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, paul.gortmaker@windriver.com On Thu, Aug 02, 2012 at 12:00:33PM +0200, Sasha Levin wrote: > On 08/02/2012 12:45 AM, Tejun Heo wrote: > > On Thu, Aug 02, 2012 at 12:41:56AM +0200, Sasha Levin wrote: > >> How would your DEFINE_HASHTABLE look like if we got for the simple > >> 'struct hash_table' approach? > > > > I think defining a different enclosing anonymous struct which the > > requested number of array entries and then aliasing the actual > > hash_table to that symbol should work. It's rather horrible and I'm > > not sure it's worth the trouble. > > I agree that this is probably not worth the trouble. > > At the moment I see two alternatives: > > 1. Dynamically allocate the hash buckets. > > 2. Use the first bucket to store size. Something like the follows: > > #define HASH_TABLE(name, bits) \ > struct hlist_head name[1 << bits + 1]; > > #define HASH_TABLE_INIT (bits) ({name[0].next = bits}); > > And then have hash_{add,get} just skip the first bucket. > > > While it's not a pretty hack, I don't see a nice way to avoid having to dynamically allocate buckets for all cases. What about using a C99 flexible array member? Kernel style prohibits variable-length arrays, but I don't think the same rationale applies to flexible array members. struct hash_table { size_t count; struct hlist_head buckets[]; }; #define DEFINE_HASH_TABLE(name, length) struct hash_table name = { .count = length, .buckets = { [0 ... (length - 1)] = HLIST_HEAD_INIT } } - Josh Triplett -- 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