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 0BE4BC4828D for ; Wed, 7 Feb 2024 12:11:47 +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=3EK7Rzcfzck9ERUY38qmsdYL1EwrXGm5LGWcf0QOEvk=; b=lE8pKIpbjyWQxV fzbnwp6S3LkXxzp7HdFmt0wwIUBQCBnMRWBhUhGiNXJ3suCQhFB/3hqh2y2AVMfToGHCZk541XCa6 j8g0JYlQgHLc3EajJyCuADH2OnVsKI0P+e9KSoNDfDcnPb3s+FW2UJpMlqlgWUKch345tyHOXI4og CnWXcYbCKesFbxvZMe1DjkribqLY9wX+e43QcS8dW+Gvj9/kQNCJo0nkuNwtQ5+AO3/WjjtFv33Rw BBveU+31crA4o/qv1OXh2XqX1YHzpNNS12Uh+7QJI393RMTIUw67mmOUVUc6FWV+OyZE4OQrQwM7U zPQt4BbwV3VZjZ+6vyCA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rXgm6-0000000Abh3-43X8; Wed, 07 Feb 2024 12:11:34 +0000 Received: from dfw.source.kernel.org ([2604:1380:4641:c500::1]) by bombadil.infradead.org with esmtps (Exim 4.97.1 #2 (Red Hat Linux)) id 1rXgm4-0000000Abfy-2sa4 for linux-arm-kernel@lists.infradead.org; Wed, 07 Feb 2024 12:11:33 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by dfw.source.kernel.org (Postfix) with ESMTP id ADFCD617C2; Wed, 7 Feb 2024 12:11:31 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 470F1C43390; Wed, 7 Feb 2024 12:11:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1707307891; bh=+eWY02fFCjdYE59uuyivWbW4kftlv5vlA77S17iwxcQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=PYCZmSdgDd7Ka4HkpDKPCOWDml3yR/9iyJOsWJqfoo0xZfG0Ba1bG6M/iB+vgF/Ek XlLLMDXXQyJyEXMCIrIWWy1LOaHDyxHMNIDH0xiwzZsN7J+Xxi12u20fRNlXOMNLxm 1HNQhaJ1v6yit9zALbj44UNRRCI2oP+uJdjhnhDJNh8SlDO75McLcDTCTaISZ8VWLL r/xknjs+QVIEOuI+O0QH8zre4TFjDT4dJUgs1dQB/L5hqomcWda890zTdMcs5dVDFJ QbdXpra5hKOnIhL/UqB/uZK2HjCTiFO6PoTGymYNhttmkZcTOkXf4ZMjqY70+eiarE eeRjS2YbwYi7A== Date: Wed, 7 Feb 2024 12:11:25 +0000 From: Will Deacon To: Matthew Wilcox Cc: 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: <20240207121125.GA22234@willie-the-truck> References: <20240113094436.2506396-1-sunnanyong@huawei.com> <20240207111252.GA22167@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240207_041132_823497_5E25C2EF X-CRM114-Status: GOOD ( 30.45 ) 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 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. 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. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel