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=-7.2 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,URIBL_BLOCKED,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 543E7C43619 for ; Thu, 29 Apr 2021 19:18:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2D4B66143A for ; Thu, 29 Apr 2021 19:18:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233792AbhD2TSA (ORCPT ); Thu, 29 Apr 2021 15:18:00 -0400 Received: from server.lespinasse.org ([63.205.204.226]:59883 "EHLO server.lespinasse.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242622AbhD2TPQ (ORCPT ); Thu, 29 Apr 2021 15:15:16 -0400 DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=lespinasse.org; i=@lespinasse.org; q=dns/txt; s=srv-14-ed; t=1619723668; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to : from; bh=nrbq9B7dG+NCQxkteDtV1okkrNphDZCfGcOZNCMSQkU=; b=2aFz00E0Y24Rhdz7Fqx5QsvnOFlMBK1oW0+IhoIU9Irj5snllxZmt2t1v5ilB6npR5/w5 gaVcVTxxmISCbNZCQ== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=lespinasse.org; i=@lespinasse.org; q=dns/txt; s=srv-14-rsa; t=1619723668; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to : from; bh=nrbq9B7dG+NCQxkteDtV1okkrNphDZCfGcOZNCMSQkU=; b=X7rNkDBMH428Ypz2fZG/kkDKqTWsYVMzTG+fA/iXVsd6nSYsDKIT4PYIS8Cme++SNHnZh gmU0NDZXd4iTNvb7sVYdFR/ZPQYQVFdMMc75zP9tCObIoNqx/s2PATVl9JF56lzXUdlkESP 7dQphM7PjcxgdcjddepwWQMWw8RZc5/PGaGiHpARm/Un9y4uGL/3ok28vtoJJgrauQaH+1f S5YqOHXFtRfvQqg8Qou7DgyKFbBh1INSxBaiIwweReMIWHs1lYBNm++XqrDmNfxv78wcs8Z jVgi867PySjlJzDpRZPFiWMLwQoW3mT+NET6t6HBddX60PBj2FmYNtwc3PDg== Received: by server.lespinasse.org (Postfix, from userid 1000) id 3E71D160305; Thu, 29 Apr 2021 12:14:28 -0700 (PDT) Date: Thu, 29 Apr 2021 12:14:28 -0700 From: Michel Lespinasse To: Matthew Wilcox Cc: Andy Lutomirski , Michel Lespinasse , "Paul E. McKenney" , Linux-MM , Laurent Dufour , Peter Zijlstra , Michal Hocko , Rik van Riel , Andrew Morton , Suren Baghdasaryan , Joel Fernandes , Rom Lemarchand , Linux-Kernel Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault(). Message-ID: <20210429191428.GD10973@lespinasse.org> References: <20210407014502.24091-1-michel@lespinasse.org> <20210407014502.24091-14-michel@lespinasse.org> <20210428145823.GA856@lespinasse.org> <20210428161108.GP975577@paulmck-ThinkPad-P17-Gen-1> <20210429000225.GC10973@lespinasse.org> <20210429161234.GG1847222@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210429161234.GG1847222@casper.infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 29, 2021 at 05:12:34PM +0100, Matthew Wilcox wrote: > On Wed, Apr 28, 2021 at 05:05:17PM -0700, Andy Lutomirski wrote: > > On Wed, Apr 28, 2021 at 5:02 PM Michel Lespinasse wrote: > > > Thanks Paul for confirming / clarifying this. BTW, it would be good to > > > add this to the rcu header files, just so people have something to > > > reference to when they depend on such behavior (like fast GUP > > > currently does). > > > > Or, even better, fast GUP could add an explicit RCU read lock. > > > > > > > > Going back to my patch. I don't need to protect against THP splitting > > > here, as I'm only handling the small page case. So when > > > MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with > > > using only an rcu read lock, instead of disabling interrupts which > > > implicitly creates the rcu read lock. I'm not sure which way to go - > > > fast GUP always disables interrupts regardless of the > > > MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be > > > made for following the fast GUP stes rather than trying to be smarter. > > > > How about adding some little helpers: > > > > lockless_page_walk_begin(); > > > > lockless_page_walk_end(); > > > > these turn into RCU read locks if MMU_GATHER_RCU_TABLE_FREE and into > > irqsave otherwise. And they're somewhat self-documenting. > > One of the worst things we can do while holding a spinlock is take a > cache miss because we then delay for several thousand cycles to wait for > the cache line. That gives every other CPU a really long opportunity > to slam into the spinlock and things go downhill fast at that point. > We've even seen patches to do things like read A, take lock L, then read > A to avoid the cache miss while holding the lock. I understand the effect your are describing, but I do not see how it applies here - what cacheline are we likely to miss on when using local_irq_disable() that we wouldn't touch if using rcu_read_lock() ? > What sort of performance effect would it have to free page tables > under RCU for all architectures? It's painful on s390 & powerpc because > different tables share the same struct page, but I have to believe that's > a solvable problem. I agree using RCU to free page tables would be a good thing to try. I am afraid of adding that to this patchset though, as it seems somewhate unrelated and adds risk. IMO we are most likely to find justification for pushing this if/when we try accessing remote mm's without taking the mmap lock, since disabling IPIs clearly wouldn't work there. -- Michel "walken" Lespinasse