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 6649CC433B4 for ; Wed, 7 Apr 2021 21:28:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 40D2961175 for ; Wed, 7 Apr 2021 21:28:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233180AbhDGV2u (ORCPT ); Wed, 7 Apr 2021 17:28:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55828 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233149AbhDGV2s (ORCPT ); Wed, 7 Apr 2021 17:28:48 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CB9C3C061761 for ; Wed, 7 Apr 2021 14:28:38 -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=otglDvep5sGkMy7KSFfJkw3zbakk63WSycFVlUg1TqE=; b=k210fC5qr7A1PRLdfR1aLQElA6 0DIe535HrWQzEyYMzcAdiU3nutuYI1KV7Kq+n3O+Qwd371xG62ipWeRoGx2yChAsT+y0tQWP50hkB pAwNSOlsY8ughxldJagntHF+qUDKQfvr68rUgIRHrytmVtWGVpXPdjw5PFInOJM0Y0hMMo+nijP2d Nx5uEWIWEFxURPhfHavS28wdP97gS9zlKx4eVpRqgjjcwy9VQn5kbvnu9pABlKZ0014LBLu3dsYMv 8/CVF2jQGOm7ZH0U5rzxv+Ey1/kaAp4IrFzZ+wFK+ryVQqCZcE8Z0CUiIQwERNb9Ggag7NjBQ2iqV enWi+B9A==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lUFhg-00F7NP-FZ; Wed, 07 Apr 2021 21:27:31 +0000 Date: Wed, 7 Apr 2021 22:27:12 +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: <20210407212712.GH2531743@casper.infradead.org> References: <20210407014502.24091-1-michel@lespinasse.org> <20210407014502.24091-25-michel@lespinasse.org> <20210407212027.GE25738@lespinasse.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210407212027.GE25738@lespinasse.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 07, 2021 at 02:20:27PM -0700, Michel Lespinasse wrote: > On Wed, Apr 07, 2021 at 04:40:34PM +0200, Peter Zijlstra wrote: > > On Tue, Apr 06, 2021 at 06:44:49PM -0700, Michel Lespinasse wrote: > > > In the speculative case, call the vm_ops->fault() method from within > > > an rcu read locked section, and verify the mmap sequence lock at the > > > start of the section. A match guarantees that the original vma is still > > > valid at that time, and that the associated vma->vm_file stays valid > > > while the vm_ops->fault() method is running. > > > > > > Note that this implies that speculative faults can not sleep within > > > the vm_ops->fault method. We will only attempt to fetch existing pages > > > from the page cache during speculative faults; any miss (or prefetch) > > > will be handled by falling back to non-speculative fault handling. > > > > > > The speculative handling case also does not preallocate page tables, > > > as it is always called with a pre-existing page table. > > > > So what's wrong with SRCU ? Laurent mumbled something about frequent > > SRCU kthread activity being a problem; is that still so and is that > > fundamentally unfixable? > > > > Because to me it seems a much more natural solution to the whole thing. > > The short answer is that I did not try SRCU. My thought process was, > page cache already uses an RCU read lock, I just need to expand its > scope a little. > > Using SRCU might allow us to hit disk during speculative faults; OTOH > we may need to switch to a more robust validation mechanism than the > global counter to reap any benefits. Why would you want to do I/O under SRCU?! The benefit of SRCU is that you can allocate page tables under SRCU. 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.