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 X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8928CC433E0 for ; Fri, 10 Jul 2020 16:58:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 4918F207BB for ; Fri, 10 Jul 2020 16:58:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="dgVJrxlU" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4918F207BB Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id B97938D0003; Fri, 10 Jul 2020 12:58:06 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B47458D0001; Fri, 10 Jul 2020 12:58:06 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A367D8D0003; Fri, 10 Jul 2020 12:58:06 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0086.hostedemail.com [216.40.44.86]) by kanga.kvack.org (Postfix) with ESMTP id 8ECFD8D0001 for ; Fri, 10 Jul 2020 12:58:06 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 242152C68 for ; Fri, 10 Jul 2020 16:58:06 +0000 (UTC) X-FDA: 77022773772.04.cent11_490a45e26ed0 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin04.hostedemail.com (Postfix) with ESMTP id EFF8D801088F for ; Fri, 10 Jul 2020 16:58:05 +0000 (UTC) X-HE-Tag: cent11_490a45e26ed0 X-Filterd-Recvd-Size: 5660 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Fri, 10 Jul 2020 16:58:05 +0000 (UTC) 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=REVCDakwgDG+DYRws7LZzQi4QLzZc//D1GvkmuJe/WI=; b=dgVJrxlUyxMQ21th+aRn2MrSte FdbzjRbZu8cLN84tlxqHdpFUtzI8O95wTMXQ/rcd+AaWG72Fv2A7dITBe7ZOMYqOkJkP5Wl2RDVQI PrmqKB+qfPCs2PhwcxeQZndW25BDLWlRghD4B9Re6xZnguBBMY+GfbMomE2r+5y9IDCsfAeIH/DmH C9EnZqB7WqxgWix+ggL/mp49WUS29EzpgQePI53Ku3ta6GNOlTwRLtiwQ46aA8MwI0KaYXr5V8FDZ QO3Rc9Rco4kbnB4D8KC0p+ua5rvu7TpfJ+SFN8HchKD8BAzsrCCyjPu/Ro1bimWM5UAQlpTmOh6dY AZQhWk4A==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jtwLK-0005Nl-8U; Fri, 10 Jul 2020 16:57:46 +0000 Date: Fri, 10 Jul 2020 17:57:46 +0100 From: Matthew Wilcox To: Andrea Arcangeli Cc: Hugh Dickins , Mike Rapoport , linux-kernel@vger.kernel.org, Alan Cox , Andrew Morton , Andy Lutomirski , Christopher Lameter , Dave Hansen , Idan Yaniv , James Bottomley , "Kirill A. Shutemov" , Peter Zijlstra , "Reshetova, Elena" , Thomas Gleixner , Tycho Andersen , linux-api@vger.kernel.org, linux-mm@kvack.org, Mike Rapoport Subject: Re: [RFC PATCH v2 1/5] mm: make HPAGE_PxD_{SHIFT,MASK,SIZE} always available Message-ID: <20200710165746.GO12769@casper.infradead.org> References: <20200706172051.19465-1-rppt@kernel.org> <20200706172051.19465-2-rppt@kernel.org> <20200710164037.GA11749@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200710164037.GA11749@redhat.com> X-Rspamd-Queue-Id: EFF8D801088F X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jul 10, 2020 at 12:40:37PM -0400, Andrea Arcangeli wrote: > Hello Hugh and Mike, > > On Mon, Jul 06, 2020 at 10:07:34PM -0700, Hugh Dickins wrote: > > Adding Andrea to Cc, he's the one who structured it that way, > > and should be consulted. > > > > I'm ambivalent myself. Many's the time I've been irritated by the > > BUILD_BUG() in HPAGE_etc, and it's responsible for very many #ifdef > > CONFIG_TRANSPARENT_HUGEPAGEs or IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)s > > that you find uglily scattered around the source. > > > > But that's the point of it: it's warning when you write code peculiar > > to THP, that is going to bloat the build of kernels without any THP. > > > > So although I've often been tempted to do as you suggest, I've always > > ended up respecting Andrea's intention, and worked around it instead > > (sometimes with #ifdef or IS_ENABLED(), sometimes with > > PMD_{SHIFT,MASK_SIZE}, sometimes with a local definition). > > The only other reasons that comes to mind in addition of optimizing > the bloat away at build time is to make it easier to identify the THP > code and to make it explicit that hugetlbfs shouldn't us it or it > could be wrong on some arches. > > However for this case the BUILD_BUG() looks right and this doesn't > look like a false positive. > > This patchset has nothing to do THP, so it'd be more correct to use > MAX_ORDER whenever the fragmentation is about the buddy (doesn't look > the case here) or PUD_SIZE/ORDER/PMD_SIZE/ORDER if the objective is > not to unnecessarily split extra and unrelated hugepud/hugepmds in the > direct mapping (as in this case). > > The real issue exposed by the BUILD_BUG is the lack of PMD_ORDER > definition and fs/dax.c already run into and it solved it locally in the > dax.c file: > > /* The order of a PMD entry */ > #define PMD_ORDER (PMD_SHIFT - PAGE_SHIFT) > > The fact it's not just this patch but also dax.c that run into the > same issue, makes me think PMD_ORDER should be defined and then you > can use PMD_* and PUD_* for this non-THP purpose. We'll run into some namespace issues. arch/arm/kernel/head.S:#define PMD_ORDER 3 arch/arm/kernel/head.S:#define PMD_ORDER 2 arch/mips/include/asm/pgtable-32.h:#define PMD_ORDER aieeee_attempt_to_allocate_pmd arch/mips/include/asm/pgtable-64.h:#define PMD_ORDER 0 arch/parisc/include/asm/pgtable.h:#define PMD_ORDER 1 /* Number of pages per pmd */ > Then the question if to remove the BUILD_BUG becomes orthogonal to > this patchset, but I don't see much value in retaining HPAGE_PMD/PUD_* > unless the BUILD_BUG is retained too, because this patchset already > hints that without the BUILD_BUG() the HPAGE_PMD_* definitions would > likely spill into non THP paths and they would lose also the only > value left (the ability to localize the THP code paths). So I wouldn't > be against removing the BUILD_BUG if it's causing maintenance > overhead, but then I would drop HPAGE_PMD_* too along with it or it > may just cause confusion. btw, using the hpage_ prefix already caused one problem in the hugetlb code: https://lore.kernel.org/linux-mm/20200629185003.97202-1-mike.kravetz@oracle.com/ I'd suggest we rename these to THP_PMD_* and THP_PUD_* to make it clear they're only for the THP case.