From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2B6BC433FE for ; Mon, 27 Sep 2021 18:13:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DAA7C60F11 for ; Mon, 27 Sep 2021 18:13:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235854AbhI0SPC (ORCPT ); Mon, 27 Sep 2021 14:15:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53910 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234406AbhI0SPB (ORCPT ); Mon, 27 Sep 2021 14:15:01 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7CDA9C061575; Mon, 27 Sep 2021 11:13:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=U3qCCDeIwDW91WNL/c36o7RQhwW5iX+T6EJcSbAQMfc=; b=a/BD8FrgoFDefLIiwWUMmYngZ8 8ggNJ42uZ34GpUiyq3ghb6jxatovwy+o87XPUppEJQ0aGBS6A1MabDY/NrRUES7HOMb8M5YG05cuL KlHPqcWSCdY1lZKAgynpKBmLcjnwE9RGzyDqsURg0wWt5ED8KwvvnoMUpKXfqr8kB5WwIdix25KpU F1n7aaLeQn76DDa5BQ+7ySzDriON+iguMkktSnIkJdT/QLQ48BlmAdeJkqhHFFiGCMN6cD65J7sfx aFU0x/cYtwWj1ieCm+WUfj3T46rEGXXybBfcElqwEIAbItVO+zsiG8WihyVfvqCGyPI2s5EGBJYRo 4AeJRfHA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mUv6x-00A466-UE; Mon, 27 Sep 2021 18:12:31 +0000 Date: Mon, 27 Sep 2021 19:12:19 +0100 From: Matthew Wilcox To: Kent Overstreet Cc: Vlastimil Babka , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Johannes Weiner , Linus Torvalds , Andrew Morton , "Darrick J. Wong" , Christoph Hellwig , David Howells Subject: Re: Struct page proposal Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 27, 2021 at 02:09:49PM -0400, Kent Overstreet wrote: > On Mon, Sep 27, 2021 at 07:05:26PM +0100, Matthew Wilcox wrote: > > On Mon, Sep 27, 2021 at 07:48:15PM +0200, Vlastimil Babka wrote: > > > On 9/23/21 03:21, Kent Overstreet wrote: > > > > So if we have this: > > > > > > > > struct page { > > > > unsigned long allocator; > > > > unsigned long allocatee; > > > > }; > > > > > > > > The allocator field would be used for either a pointer to slab/slub's state, if > > > > it's a slab page, or if it's a buddy allocator page it'd encode the order of the > > > > allocation - like compound order today, and probably whether or not the > > > > (compound group of) pages is free. > > > > > > The "free page in buddy allocator" case will be interesting to implement. > > > What the buddy allocator uses today is: > > > > > > - PageBuddy - determine if page is free; a page_type (part of mapcount > > > field) today, could be a bit in "allocator" field that would have to be 0 in > > > all other "page is allocated" contexts. > > > - nid/zid - to prevent merging accross node/zone boundaries, now part of > > > page flags > > > - buddy order > > > - a list_head (reusing the "lru") to hold the struct page on the appropriate > > > free list, which has to be double-linked so page can be taken from the > > > middle of the list instantly > > > > > > Won't be easy to cram all that into two unsigned long's, or even a single > > > one. We should avoid storing anything in the free page itself. Allocating > > > some external structures to track free pages is going to have funny > > > bootstrap problems. Probably a major redesign would be needed... > > > > Wait, why do we want to avoid using the memory that we're allocating? > > The issue is where to stick the state for free pages. If that doesn't fit in two > ulongs, then we'd need a separate allocation, which means slab needs to be up > and running before free pages are initialized. But the thing we're allocating is at least PAGE_SIZE bytes in size. Why is "We should avoid storing anything in the free page itself" true?