From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id 3C11D6B004D for ; Thu, 1 Oct 2009 06:19:21 -0400 (EDT) Date: Thu, 1 Oct 2009 11:54:34 +0100 (BST) From: Hugh Dickins Subject: Re: No more bits in vm_area_struct's vm_flags. In-Reply-To: Message-ID: References: <4AB9A0D6.1090004@crca.org.au> <20090924100518.78df6b93.kamezawa.hiroyu@jp.fujitsu.com> <4ABC80B0.5010100@crca.org.au> <20090925174009.79778649.kamezawa.hiroyu@jp.fujitsu.com> <4AC0234F.2080808@crca.org.au> <20090928120450.c2d8a4e2.kamezawa.hiroyu@jp.fujitsu.com> <20090928033624.GA11191@localhost> <20090928125705.6656e8c5.kamezawa.hiroyu@jp.fujitsu.com> <20090929105735.06eea1ee.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Christoph Lameter Cc: KAMEZAWA Hiroyuki , Wu Fengguang , Nigel Cunningham , LKML , "linux-mm@kvack.org" List-ID: On Tue, 29 Sep 2009, Christoph Lameter wrote: > > Another concern that has not been discussed is the increased cache > footprint due to a slightly enlarged vm data working set (there is also a > corresponding icache issue since additional accesses are needed). Using unsigned long long vm_flags makes no difference to cache footprint on 64-bit systems, being a no-op there; and I think these days, though we sure like our 32-bit systems to run well, we're not so anxious about saving every last cycle on them. > > Could we stick with the current size and do combinations of flags like we > do with page flags? Are we doing that? If you have some example like, when PG_slab is set then PG_owner_priv_1 means such-and-such, but if not not: okay, I'm fine with that. But if you're saying something like, if PG_reclaim is set at the same time as PG_buddy, then they mean the page is not a buddy or under reclaim, but brokenbacked: then I'm a bit (or even 32 bits) worried. > VM_HUGETLB cannot grow up and down f.e. and there are > certainly lots of other impossible combinations that can be used to put > more information into the flags. Where it makes sense, where it's understandable, okay: there may be a few which could naturally use combinations. But in general, no, I think we'd be asking for endless maintenance trouble if we change the meaning of some flags according to other flags. Hugh -- 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