From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761337AbXJOXfe (ORCPT ); Mon, 15 Oct 2007 19:35:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754890AbXJOXf0 (ORCPT ); Mon, 15 Oct 2007 19:35:26 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:44181 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754899AbXJOXfZ (ORCPT ); Mon, 15 Oct 2007 19:35:25 -0400 Subject: Re: [PATCH 10/11] maps3: add /proc/kpagecount and /proc/kpageflags interfaces From: Dave Hansen To: Matt Mackall Cc: Andrew Morton , linux-kernel@vger.kernel.org, Rusty Russell , Jeremy Fitzhardinge , David Rientjes , Fengguang Wu In-Reply-To: <20071015231106.GX19691@waste.org> References: <11.290135367@selenic.com> <1192488513.6118.98.camel@localhost> <20071015231106.GX19691@waste.org> Content-Type: text/plain Date: Mon, 15 Oct 2007 16:34:57 -0700 Message-Id: <1192491297.6118.129.camel@localhost> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2007-10-15 at 18:11 -0500, Matt Mackall wrote: > > Could we just have /proc/kpagereferenced? Is there a legitimate need > > for other flags to be visible? > > Referenced, dirty, uptodate, lru, active, slab, writeback, reclaim, > and buddy all look like they might be interesting to me from the point > of view of watching what's happening in the VM graphically in > real-time. This is true, but it forces a lot of logic from the kernel to be run in userspace to figure out what is going on. Looking at mainline today: #define PG_reclaim 17 /* To be reclaimed asap */ ... #define PG_readahead PG_reclaim /* Reminder to do async read-ahead */ All of a sudden, to figure out which flag it actually is, we need to have all of the logic that the kernel does. Does this establish a fixed user<->kernel ABI that will keep us from doing this in the future: -#define PG_slab 7 /* slab debug (Suparna wants this) */ +#define PG_slab 14 /* slab debug (Suparna wants this) */ Or, even something like this: -#define PageSlab(page) test_bit(PG_slab, &(page)->flags) +#define PageSlab(page) (!PageLRU(page) && !PageHighmem(page)) If we actually had several (or even still one file) that exposed this state, independent of the actual content of page->flags, I think we'd be better off. I think that's the difference between a fun, super-useful debugging feature and one that can stay in mainline and have applications stay using it (without breaking) for a long time. The flags you listed are things that I would imagine will always exist, logically. But, we might not always have a specific page flag for pages under writeback or in the buddy list for that matter. PG_buddy isn't that old. Perhaps that would be better abstracted to something like page_in_main_allocator(). -- Dave