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 X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 18580C4360C for ; Wed, 16 Oct 2019 06:42:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D8F1720873 for ; Wed, 16 Oct 2019 06:42:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=shipmail.org header.i=@shipmail.org header.b="pPPZbgm6" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388047AbfJPGmX (ORCPT ); Wed, 16 Oct 2019 02:42:23 -0400 Received: from ste-pvt-msa2.bahnhof.se ([213.80.101.71]:57741 "EHLO ste-pvt-msa2.bahnhof.se" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728378AbfJPGmX (ORCPT ); Wed, 16 Oct 2019 02:42:23 -0400 Received: from localhost (localhost [127.0.0.1]) by ste-pvt-msa2.bahnhof.se (Postfix) with ESMTP id 7FDE33F9BD; Wed, 16 Oct 2019 08:42:15 +0200 (CEST) Authentication-Results: ste-pvt-msa2.bahnhof.se; dkim=pass (1024-bit key; unprotected) header.d=shipmail.org header.i=@shipmail.org header.b=pPPZbgm6; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at bahnhof.se Authentication-Results: ste-ftg-msa2.bahnhof.se (amavisd-new); dkim=pass (1024-bit key) header.d=shipmail.org Received: from ste-pvt-msa2.bahnhof.se ([127.0.0.1]) by localhost (ste-ftg-msa2.bahnhof.se [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id S_wGHDoaqxPm; Wed, 16 Oct 2019 08:42:14 +0200 (CEST) Received: from mail1.shipmail.org (h-205-35.A357.priv.bahnhof.se [155.4.205.35]) (Authenticated sender: mb878879) by ste-pvt-msa2.bahnhof.se (Postfix) with ESMTPA id F27B33F9AA; Wed, 16 Oct 2019 08:42:12 +0200 (CEST) Received: from localhost.localdomain (h-205-35.A357.priv.bahnhof.se [155.4.205.35]) by mail1.shipmail.org (Postfix) with ESMTPSA id 2B0F236016A; Wed, 16 Oct 2019 08:42:12 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=shipmail.org; s=mail; t=1571208132; bh=mWx5XiZ18RyCNn31L5t+RJV/zV42tlypsAdDAf2qsTY=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=pPPZbgm6n3Xs7qxcyyTkGfHL9NTkrulg+YkkNIGgcLhHXkWoAmcqKvUNk6wQ4pR9v o1C/C2B2NoGNs9hMB11UgywSYcpY0sswuSj9vx2doiPx8g0/8eV6umhLQWgBLh+SXF FVyCpoJ/S0MMm3R371V9Bj4n10SM7r58S4kum3l8= Subject: Re: [PATCH v5 4/8] mm: Add write-protect and clean utilities for address space ranges To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, torvalds@linux-foundation.org, kirill@shutemov.name, Thomas Hellstrom , Andrew Morton , Matthew Wilcox , Will Deacon , Rik van Riel , Minchan Kim , Michal Hocko , Huang Ying , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= References: <20191010124314.40067-1-thomas_os@shipmail.org> <20191010124314.40067-5-thomas_os@shipmail.org> <20191010130542.GP2328@hirez.programming.kicks-ass.net> <45cf5965-bd63-3574-d8c2-abbd6c4960d5@shipmail.org> <20191010141708.GV2311@hirez.programming.kicks-ass.net> From: =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28VMware=29?= Organization: VMware Inc. Message-ID: <7594be1d-01d5-62b7-8947-022f9ebeb845@shipmail.org> Date: Wed, 16 Oct 2019 08:42:11 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20191010141708.GV2311@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/10/19 4:17 PM, Peter Zijlstra wrote: > On Thu, Oct 10, 2019 at 03:24:47PM +0200, Thomas Hellström (VMware) wrote: >> On 10/10/19 3:05 PM, Peter Zijlstra wrote: >>> On Thu, Oct 10, 2019 at 02:43:10PM +0200, Thomas Hellström (VMware) wrote: >>>> +/** >>>> + * wp_shared_mapping_range - Write-protect all ptes in an address space range >>>> + * @mapping: The address_space we want to write protect >>>> + * @first_index: The first page offset in the range >>>> + * @nr: Number of incremental page offsets to cover >>>> + * >>>> + * Note: This function currently skips transhuge page-table entries, since >>>> + * it's intended for dirty-tracking on the PTE level. It will warn on >>>> + * encountering transhuge write-enabled entries, though, and can easily be >>>> + * extended to handle them as well. >>>> + * >>>> + * Return: The number of ptes actually write-protected. Note that >>>> + * already write-protected ptes are not counted. >>>> + */ >>>> +unsigned long wp_shared_mapping_range(struct address_space *mapping, >>>> + pgoff_t first_index, pgoff_t nr) >>>> +{ >>>> + struct wp_walk wpwalk = { .total = 0 }; >>>> + >>>> + i_mmap_lock_read(mapping); >>>> + WARN_ON(walk_page_mapping(mapping, first_index, nr, &wp_walk_ops, >>>> + &wpwalk)); >>>> + i_mmap_unlock_read(mapping); >>>> + >>>> + return wpwalk.total; >>>> +} >>> That's a read lock, this means there's concurrency to self. What happens >>> if someone does two concurrent wp_shared_mapping_range() on the same >>> mapping? >>> >>> The thing is, because of pte_wrprotect() the iteration that starts last >>> will see a smaller pte_write range, if it completes first and does >>> flush_tlb_range(), it will only flush a partial range. >>> >>> This is exactly what {inc,dec}_tlb_flush_pending() is for, but you're >>> not using mm_tlb_flush_nested() to detect the situation and do a bigger >>> flush. >>> >>> Or if you're not needing that, then I'm missing why. >> Good catch. Thanks, >> >> Yes the read lock is not intended to protect against concurrent users but to >> protect the vmas from disappearing under us. Since it fundamentally makes no >> sense having two concurrent threads picking up dirty ptes on the same >> address_space range we have an external range-based lock to protect against >> that. > Nothing mandates/verifies the function you expose is used exclusively. > Therefore you cannot make assumptions on that range lock your user has. > >> However, that external lock doesn't protect other code  from concurrently >> modifying ptes and having the mm's  tlb_flush_pending increased, so I guess >> we unconditionally need to test for that and do a full range flush if >> necessary? > Yes, something like: > > if (mm_tlb_flush_nested(mm)) > flush_tlb_range(walk->vma, walk->vma->vm_start, walk->vma->vm_end); > else if (wpwalk->tlbflush_end > wpwalk->tlbflush_start) > flush_tlb_range(walk->vma, wpwalk->tlbflush_start, wpwalk->tlbflush_end); > Hi, Peter, I've updated the patch to incorporate something similar to the above. Since you've looked at the patch, any chance of an R-B? Thanks, Thomas