From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx164.postini.com [74.125.245.164]) by kanga.kvack.org (Postfix) with SMTP id 992406B004D for ; Thu, 2 Aug 2012 07:23:12 -0400 (EDT) Received: by bkcjc3 with SMTP id jc3so5074798bkc.14 for ; Thu, 02 Aug 2012 04:23:10 -0700 (PDT) Message-ID: <501A633B.3010509@gmail.com> Date: Thu, 02 Aug 2012 13:23:39 +0200 From: Sasha Levin MIME-Version: 1.0 Subject: Re: [RFC 1/4] hashtable: introduce a small and naive hashtable 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> <20120802103244.GA23318@leaf> In-Reply-To: <20120802103244.GA23318@leaf> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Josh Triplett 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 08/02/2012 12:32 PM, Josh Triplett wrote: > 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 } } The limitation of this approach is that the struct hash_table variable must be 'static', which is a bit limiting - see for example the use of hashtable in 'struct user_namespace'. > > - 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