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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 61772C4828F for ; Wed, 7 Feb 2024 12:24:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=tdCNfayeGxOiStsQVSzf6bhv9mEYZUOMfi1r4IMw5XQ=; b=REr9apFtRt+O/P fM9LO8TQzD366aMVLQWTtBFNFvRRf5BaYZaDEnKcds0xpkth0oPDja81JsgmTgmKRY1yahMu4DQz3 NkUf/Fp3xuTpBgkYKzVdbBKoMO+lBfdPXlHEqDW221f/rRR6wsgUZAj+DVOUbe23F3iTHHK1iGqR9 eELbfChhncAaiFGWA5vS4IBYqlwVOQ8mphmOvVtOMvNHH8cRBLpyCCH4gzHs1zwtnMRMBrNlOcgG9 yOb81fB+SYzw+c8vf1a8snu9dODv1KxgmgEGlExY0mQmPauWrAEDNEOBe1U+/ZAhOJwaERt9sMIem /0fnbGxkNsCyP6BFC9cg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rXgyi-0000000AdBF-0G95; Wed, 07 Feb 2024 12:24:36 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rXgyf-0000000Ad9K-0NZh for linux-arm-kernel@lists.infradead.org; Wed, 07 Feb 2024 12:24:34 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 754951FB; Wed, 7 Feb 2024 04:25:13 -0800 (PST) Received: from FVFF77S0Q05N.cambridge.arm.com (FVFF77S0Q05N.cambridge.arm.com [10.1.26.150]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 750803F5A1; Wed, 7 Feb 2024 04:24:29 -0800 (PST) Date: Wed, 7 Feb 2024 12:24:26 +0000 From: Mark Rutland To: Will Deacon Cc: Matthew Wilcox , Nanyong Sun , Catalin Marinas , mike.kravetz@oracle.com, muchun.song@linux.dev, akpm@linux-foundation.org, anshuman.khandual@arm.com, wangkefeng.wang@huawei.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize Message-ID: References: <20240113094436.2506396-1-sunnanyong@huawei.com> <20240207111252.GA22167@willie-the-truck> <20240207121125.GA22234@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20240207121125.GA22234@willie-the-truck> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240207_042433_253453_2E5AB7CA X-CRM114-Status: GOOD ( 34.78 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Feb 07, 2024 at 12:11:25PM +0000, Will Deacon wrote: > On Wed, Feb 07, 2024 at 11:21:17AM +0000, Matthew Wilcox wrote: > > On Wed, Feb 07, 2024 at 11:12:52AM +0000, Will Deacon wrote: > > > On Sat, Jan 27, 2024 at 01:04:15PM +0800, Nanyong Sun wrote: > > > > > > > > On 2024/1/26 2:06, Catalin Marinas wrote: > > > > > On Sat, Jan 13, 2024 at 05:44:33PM +0800, Nanyong Sun wrote: > > > > > > HVO was previously disabled on arm64 [1] due to the lack of necessary > > > > > > BBM(break-before-make) logic when changing page tables. > > > > > > This set of patches fix this by adding necessary BBM sequence when > > > > > > changing page table, and supporting vmemmap page fault handling to > > > > > > fixup kernel address translation fault if vmemmap is concurrently accessed. > > > > > I'm not keen on this approach. I'm not even sure it's safe. In the > > > > > second patch, you take the init_mm.page_table_lock on the fault path but > > > > > are we sure this is unlocked when the fault was taken? > > > > I think this situation is impossible. In the implementation of the second > > > > patch, when the page table is being corrupted > > > > (the time window when a page fault may occur), vmemmap_update_pte() already > > > > holds the init_mm.page_table_lock, > > > > and unlock it until page table update is done.Another thread could not hold > > > > the init_mm.page_table_lock and > > > > also trigger a page fault at the same time. > > > > If I have missed any points in my thinking, please correct me. Thank you. > > > > > > It still strikes me as incredibly fragile to handle the fault and trying > > > to reason about all the users of 'struct page' is impossible. For example, > > > can the fault happen from irq context? > > > > The pte lock cannot be taken in irq context (which I think is what > > you're asking?) While it is not possible to reason about all users of > > struct page, we are somewhat relieved of that work by noting that this is > > only for hugetlbfs, so we don't need to reason about slab, page tables, > > netmem or zsmalloc. > > My concern is that an interrupt handler tries to access a 'struct page' > which faults due to another core splitting a pmd mapping for the vmemmap. > In this case, I think we'll end up trying to resolve the fault from irq > context, which will try to take the spinlock. I think that (as per my comments on patch 2), a similar deadlock can happen on RT even if the vmemmap is only accessed in regular process context, and at minimum this needs better comentary and/or lockdep assertions. I'd also prefer that we dropped this for now. > Avoiding the fault would make this considerably more robust and the > architecture has introduced features to avoid break-before-make in some > circumstances (see FEAT_BBM and its levels), so having this optimisation > conditional on that would seem to be a better approach in my opinion. FWIW, that's my position too. Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel