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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 73695C6FD20 for ; Fri, 24 Mar 2023 14:54:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232261AbjCXOyY (ORCPT ); Fri, 24 Mar 2023 10:54:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51106 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232245AbjCXOyU (ORCPT ); Fri, 24 Mar 2023 10:54:20 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6AD372D5F for ; Fri, 24 Mar 2023 07:53:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1679669609; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=uVxo6T8zn6ArXhplZHuhgmR3CWjjWZDjou2/HtzSxk0=; b=D6J/IgjRlNAbbAafTuTJscdB7tA8Yct9QpGC2JDUDVG3UU0b7HrNccAfPNsfcWL0zlSZeL CFDUB/1jPUN9dwebQfu2OuXUvoQ5Lnax/y11qhcs6X8BM2+luQrNqow4rLIYQJ7SEEr09O y9HQcrw6IwC3bYTNIiAsnhTmPp96lak= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-633-vzeQFkzOOqWYN1Ei5YpV6g-1; Fri, 24 Mar 2023 10:53:23 -0400 X-MC-Unique: vzeQFkzOOqWYN1Ei5YpV6g-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 339AA85A5B1; Fri, 24 Mar 2023 14:53:22 +0000 (UTC) Received: from localhost (ovpn-12-55.pek2.redhat.com [10.72.12.55]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D540540C83AC; Fri, 24 Mar 2023 14:53:16 +0000 (UTC) Date: Fri, 24 Mar 2023 22:53:12 +0800 From: Baoquan He To: "Leizhen (ThunderTown)" Cc: Catalin Marinas , linux-kernel@vger.kernel.org, horms@kernel.org, John.p.donnelly@oracle.com, will@kernel.org, kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v4] arm64: kdump: simplify the reservation behaviour of crashkernel=,high Message-ID: References: <20230306084124.300584-1-bhe@redhat.com> <4d4ecdd6-9716-570d-5595-e47bfbb58cdf@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4d4ecdd6-9716-570d-5595-e47bfbb58cdf@huawei.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Leizhen, On 03/24/23 at 10:47am, Leizhen (ThunderTown) wrote: ...... > >>>> 2) with the fixed CRASH_ADDR_LOW_MAX as 4G, we can easily fix the > >>>> problem of base page mapping for the whole linear mapping if crsahkernel= > >>>> is set in kernel parameter shown in [1] at bottom. > >>> > >>> That's a different problem ;). I should re-read that thread, forgot most > >>> of the details but I recall one of the counter arguments was that there > >>> isn't a strong case to unmap the crashkernel reservation. Now, if we > >>> place crashdump kernel image goes in the 'high' reservation, can we not > >>> leave the 'low' reservation mapped? We don't really care about it as it > >>> wouldn't have any meaningful code/data to be preserved. If the 'high' > >>> one goes above 4G always, we don't depend on the arm64_dma_phys_limit. > >> > >> Yes, this looks ideal. While it only works when crashkernel=,high case and > >> it succeeds to reserve a memory region for the specified size of crashkernel > >> high memory. At below, we have 4 cases of crashkernel= syntax: > >> > >> crashkernel=size > >> 1)first attempt: low memory under arm64_dma_phys_limit > >> 2)fallback: finding memory above 4G > > > > (2) should be 'finding memory above arm64_dma_phys_limit' to keep the > > current behaviour for RPi4. > > > >> crashkernel=size,high > >> 3)first attempt: finding memory above 4G > >> 4)fallback: low memory under arm64_dma_phys_limit > > > > Yes. > > > >> case 3) works with your suggestion. However, 1), 2), 4) all need to > >> defer to bootmem_init(). With these cases and different handling, > >> reserve_crashkernel() could be too complicated. > > > > Ah, because of the fallback below arm64_dma_phys_limit as in (4), we > > still can't move the full crashkernel reservation early. Well, we could > > do it in two steps: (a) early attempt at crashkernel reservation above > > 4G if 'high' was specified and we avoid mapping it if successful and (b) > > do the late crashkernel reservation below arm64_dma_phys_limit and skip > > unmapping as being too late. This way most server-like platforms would > > get a reservation above 4G, unmapped. > > > >> I am wondering if we can cancel the protection of crashkernel memory > >> region on arm64 for now. In earlier discussion, people questioned if the > >> protection is necessary on arm64. After comparison, I would rather take > >> away the protection method of crashkernel region since they try to > >> protect in a chance in one million , while the base page mapping for the > >> whole linear mapping is mitigating arm64 high end server always. > > > > This works for me. We can add the protection later for addresses above > > 4GB only as mentioned above. > > Recently, I've also been rethinking the performance issues when kdump is > enabled. I have a new idea. For crashkernel=X, we can temporarily search > for free memory from the low address to the high address. As below: > > save_bottom_up = memblock_bottom_up(); > if (!high) > memblock_set_bottom_up(true); > crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN, crash_base, crash_max); > memblock_set_bottom_up(save_bottom_up); > > The final code change should be small, and I'll try it today. I have sent a patchset to remove the crashkernel region protection code as per Catalin's confirmation. I personally like the code conciseness w/o protection because kinds of crahskernel reservation has been complex, the situation on arm64 will makes it worse if we try to keep the protection and fix the performance issue. While I am glad to see any attempt to achieve the two goals if it's satisfactory.