Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752055AbdFMEw4 (ORCPT ); Tue, 13 Jun 2017 00:52:56 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:49671 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751903AbdFMEwz (ORCPT ); Tue, 13 Jun 2017 00:52:55 -0400 From: "Aneesh Kumar K.V" To: Ram Pai , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Cc: benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au, khandual@linux.vnet.ibm.com, bsingharora@gmail.com, dave.hansen@intel.com, hbabu@us.ibm.com, linuxram@us.ibm.com Subject: Re: [RFC PATCH 1/7 v1]powerpc: Free up four PTE bits to accommodate memory keys In-Reply-To: <1496711109-4968-2-git-send-email-linuxram@us.ibm.com> References: <1496711109-4968-1-git-send-email-linuxram@us.ibm.com> <1496711109-4968-2-git-send-email-linuxram@us.ibm.com> Date: Tue, 13 Jun 2017 10:22:43 +0530 MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-MML: disable x-cbid: 17061304-0040-0000-0000-0000032CDB38 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17061304-0041-0000-0000-00000CA6FC34 Message-Id: <87tw3k1obo.fsf@skywalker.in.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-06-13_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=5 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1706130082 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2038 Lines: 51 Ram Pai writes: > Rearrange PTE bits to free up bits 3, 4, 5 and 6 for > memory keys. Bit 3, 4, 5, 6 and 57 shall be used for memory > keys. > > The patch does the following change to the 64K PTE format > > H_PAGE_BUSY moves from bit 3 to bit 7 > H_PAGE_F_SECOND which occupied bit 4 moves to the second part > of the pte. > H_PAGE_F_GIX which occupied bit 5, 6 and 7 also moves to the > second part of the pte. > > The second part of the PTE will hold > a (H_PAGE_F_SECOND|H_PAGE_F_GIX) for 64K page backed pte, > and sixteen (H_PAGE_F_SECOND|H_PAGE_F_GIX) for 4k backed > pte. > > the four bits((H_PAGE_F_SECOND|H_PAGE_F_GIX) that represent a slot > is initialized to 0xF indicating a invalid slot. if a hashpage does > get allocated to the 0xF slot, it is released and not used. In > other words, even though 0xF is a valid slot we discard it and > consider it as invalid slot(HPTE_SOFT_INVALID). This gives us an > opportunity to not depend on a bit in the primary PTE in order to > determine the validity of a slot. > > When we release a 0xF slot we also release a legitimate primary > slot and unmap that entry. This is to ensure that we do get > a legimate non-0xF slot the next time we retry for a slot. > > Though treating 0xF slot as invalid reduces the number of available > slots and make have a effect on the performance, the probabilty > of hitting a 0xF is extermely low. > > Compared to the current scheme, the above described scheme reduces > the number of false hash table updates significantly and has the > added advantage of releasing four valuable PTE bits for other > purpose. > > This idea was jointly developed by Paul Mackerras, Aneesh, Michael > Ellermen and myself. > > 4K PTE format remain unchanged currently. > Can you also split this patch into two. One which changes __hash_page_4k() ie, linux pte format w.r.t 4k hash pte. Second patch with changes w.r.t __hash_page_64k() ie, pte format w.r.t 64k hash pte. -aneesh