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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 B5551C433B4 for ; Thu, 8 Apr 2021 11:29:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6BC1161105 for ; Thu, 8 Apr 2021 11:29:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229721AbhDHL3N (ORCPT ); Thu, 8 Apr 2021 07:29:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40286 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230443AbhDHL3J (ORCPT ); Thu, 8 Apr 2021 07:29:09 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5D085C061760 for ; Thu, 8 Apr 2021 04:28:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=6L5DXhJLDb/LJ3Zk1Inyo0+MRzl97ueT3hWzUgjTxAQ=; b=U2ol3/AFz4trLaB07jh5OLdQJs Q1M4/k50bMUB2/GQPRrO766TWjKMaleVHYyan0480tHf8jxJRWfyjALxMjEJ8iK8sK1JFnKougkzd tGh+2PtppBE0x3iBq7JxyYPTex/pKkyXgBnKw1TIewUOEg19kzQqeBqgh6HTk5KK5J4UvYpsCul30 frBeyv5A0WEe6wn2OoCFeDyxb6KM6AApyiG6/6hBqS19FSQApVAU1bFZNVNccLzSGk+WemouTpvf5 1dWiS3riBVJeDgFgJ0xbkQCT5c0ghR5sipnBFP4p6FQYvYSuCrUNy9N2y51ChGKN0SPbaSDX5mz41 eupWbCxw==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lUSpU-00G52T-SR; Thu, 08 Apr 2021 11:28:10 +0000 Date: Thu, 8 Apr 2021 12:28:08 +0100 From: Matthew Wilcox To: Michel Lespinasse Cc: Peter Zijlstra , Linux-MM , Laurent Dufour , Michal Hocko , Rik van Riel , Paul McKenney , Andrew Morton , Suren Baghdasaryan , Joel Fernandes , Rom Lemarchand , Linux-Kernel Subject: Re: [RFC PATCH 24/37] mm: implement speculative handling in __do_fault() Message-ID: <20210408112808.GK2531743@casper.infradead.org> References: <20210407014502.24091-1-michel@lespinasse.org> <20210407014502.24091-25-michel@lespinasse.org> <20210407212027.GE25738@lespinasse.org> <20210407212712.GH2531743@casper.infradead.org> <20210408071343.GJ2531743@casper.infradead.org> <20210408083734.GB27824@lespinasse.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210408083734.GB27824@lespinasse.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 08, 2021 at 01:37:34AM -0700, Michel Lespinasse wrote: > On Thu, Apr 08, 2021 at 08:13:43AM +0100, Matthew Wilcox wrote: > > On Thu, Apr 08, 2021 at 09:00:26AM +0200, Peter Zijlstra wrote: > > > On Wed, Apr 07, 2021 at 10:27:12PM +0100, Matthew Wilcox wrote: > > > > Doing I/O without any lock held already works; it just uses the file > > > > refcount. It would be better to use a vma refcount, as I already said. > > > > > > The original workload that I developed SPF for (waaaay back when) was > > > prefaulting a single huge vma. Using a vma refcount was a total loss > > > because it resulted in the same cacheline contention that down_read() > > > was having. > > > > > > As such, I'm always incredibly sad to see mention of vma refcounts. > > > They're fundamentally not solving the problem :/ > > > > OK, let me outline my locking scheme because I think it's rather better > > than Michel's. The vma refcount is the slow path. > > > > 1. take the RCU read lock > > 2. walk the pgd/p4d/pud/pmd > > 3. allocate page tables if necessary. *handwave GFP flags*. > > 4. walk the vma tree > > 5. call ->map_pages > > 6. take ptlock > > 7. insert page(s) > > 8. drop ptlock > > if this all worked out, we're done, drop the RCU read lock and return. > > 9. increment vma refcount > > 10. drop RCU read lock > > 11. call ->fault > > 12. decrement vma refcount > > Note that most of your proposed steps seem similar in principle to mine. > Looking at the fast path (steps 1-8): > - step 2 sounds like the speculative part of __handle_mm_fault() > - (step 3 not included in my proposal) > - step 4 is basically the lookup I currently have in the arch fault handler > - step 6 sounds like the speculative part of map_pte_lock() > > I have working implementations for each step, while your proposal > summarizes each as a point item. It's not clear to me what to make of it; > presumably you would be "filling in the blanks" in a different way > than I have but you are not explaining how. Are you suggesting that > the precautions taken in each step to avoid races with mmap writers > would not be necessary in your proposal ? if that is the case, what is > the alternative mechanism would you use to handle such races ? I don't know if you noticed, I've been a little busy with memory folios. I did tell you that on the call, but you don't seem to retain anything I tell you on the call, so maybe I shouldn't bother calling in any more. > Going back to the source of this, you suggested not copying the VMA, > what is your proposed alternative ? Do you suggest that fault handlers > should deal with the vma potentially mutating under them ? Or should > mmap writers consider vmas as immutable and copy them whenever they > want to change them ? or are you implying a locking mechanism that would > prevent mmap writers from executing while the fault is running ? The VMA should be immutable, as I explained to you before.