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 D65E6C43334 for ; Tue, 21 Jun 2022 18:05:36 +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=rqQAViB8ds2OX/oUFn8VIkV1l6q1ulj5syOZeo6tUAA=; b=LMfz2FON+PyZXd mI4Etg8zCzzWYaXQtYRzWotxgkANW7FN/XhwR6nG1g9uhVGhuJTJsXwB5Ix0F3xll6NLgquHSVTak g70pMjevyBhJP2I46lsOUIt2K4CViuK00EqJGYRHGAPWiHTDZrwqsLn8r7G9W4T47M0za0RluPSZH 7t2O/Au/+NAqk0q2foqUsYdto7Lgl6y7khLQFo+G2U84jnZ8OURKnBoaYLFrSrKB5IRM4qzsxjggq Vo7lLgqImPUmolwDIoYfPgXg+tUgpUV8ou737M6qfPciy8qV+Ktm8OlBlEsTWJRZFIxqzrpfqztHl XyDbisBC0nBxjl6CmulQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1o3iEw-006fcV-33; Tue, 21 Jun 2022 18:04:38 +0000 Received: from ams.source.kernel.org ([145.40.68.75]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1o3iEr-006faf-7C; Tue, 21 Jun 2022 18:04:35 +0000 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 97936B81AAC; Tue, 21 Jun 2022 18:04:31 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9A105C3411C; Tue, 21 Jun 2022 18:04:25 +0000 (UTC) Date: Tue, 21 Jun 2022 19:04:22 +0100 From: Catalin Marinas To: Kefeng Wang Cc: Baoquan He , Zhen Lei , Ard Biesheuvel , Mark Rutland , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Eric Biederman , Rob Herring , Frank Rowand , devicetree@vger.kernel.org, Dave Young , Vivek Goyal , kexec@lists.infradead.org, linux-kernel@vger.kernel.org, Will Deacon , linux-arm-kernel@lists.infradead.org, Jonathan Corbet , linux-doc@vger.kernel.org, Randy Dunlap , Feng Zhou , Chen Zhou , John Donnelly , Dave Kleikamp , liushixin Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory Message-ID: References: <20220613080932.663-1-thunder.leizhen@huawei.com> <20220613080932.663-6-thunder.leizhen@huawei.com> <3f66323d-f371-b931-65fb-edfae0f01c88@huawei.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <3f66323d-f371-b931-65fb-edfae0f01c88@huawei.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220621_110433_585929_06EE6562 X-CRM114-Status: GOOD ( 33.02 ) 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="iso-8859-1" Content-Transfer-Encoding: quoted-printable Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jun 21, 2022 at 02:24:01PM +0800, Kefeng Wang wrote: > On 2022/6/21 13:33, Baoquan He wrote: > > On 06/13/22 at 04:09pm, Zhen Lei wrote: > > > If the crashkernel has both high memory above DMA zones and low memory > > > in DMA zones, kexec always loads the content such as Image and dtb to= the > > > high memory instead of the low memory. This means that only high memo= ry > > > requires write protection based on page-level mapping. The allocation= of > > > high memory does not depend on the DMA boundary. So we can reserve the > > > high memory first even if the crashkernel reservation is deferred. > > > = > > > This means that the block mapping can still be performed on other ker= nel > > > linear address spaces, the TLB miss rate can be reduced and the system > > > performance will be improved. > > = > > Ugh, this looks a little ugly, honestly. > > = > > If that's for sure arm64 can't split large page mapping of linear > > region, this patch is one way to optimize linear mapping. Given kdump > > setting is necessary on arm64 server, the booting speed is truly > > impacted heavily. > = > Is there some conclusion or discussion that arm64 can't split large page > mapping? > = > Could the crashkernel reservation (and Kfence pool) be splited dynamicall= y? > = > I found Mark replay "arm64: remove page granularity limitation from > KFENCE"[1], > = > =A0 "We also avoid live changes from block<->table mappings, since the > =A0 archtitecture gives us very weak guarantees there and generally requi= res > =A0 a Break-Before-Make sequence (though IIRC this was tightened up > =A0 somewhat, so maybe going one way is supposed to work). Unless it's > =A0 really necessary, I'd rather not split these block mappings while > =A0 they're live." The problem with splitting is that you can end up with two entries in the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict abort (but can be worse like loss of coherency). Prior to FEAT_BBM (added in ARMv8.4), such scenario was not allowed at all, the software would have to unmap the range, TLBI, remap. With FEAT_BBM (level 2), we can do this without tearing the mapping down but we still need to handle the potential TLB conflict abort. The handler only needs a TLBI but if it touches the memory range being changed it risks faulting again. With vmap stacks and the kernel image mapped in the vmalloc space, we have a small window where this could be handled but we probably can't go into the C part of the exception handling (tracing etc. may access a kmalloc'ed object for example). Another option is to do a stop_machine() (if multi-processor at that point), disable the MMUs, modify the page tables, re-enable the MMU but it's also complicated. -- = Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel