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=-5.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS, USER_AGENT_MUTT autolearn=ham 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 175C1C169C4 for ; Mon, 11 Feb 2019 17:57:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DCE7C21B68 for ; Mon, 11 Feb 2019 17:57:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="O9Z8ZFPK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729830AbfBKR5A (ORCPT ); Mon, 11 Feb 2019 12:57:00 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:41348 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726104AbfBKR5A (ORCPT ); Mon, 11 Feb 2019 12:57:00 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=kI0sdjrum5jiu2PQ9Dll1y9K5Hr4fyaJQ95yFl7qdOI=; b=O9Z8ZFPKkq/iHXeziHspJFmWG qVV4bCOqDdmhhIFhQMuGKu+TYyI2ZfgdNo2CMjMBd5jLYMfe5rwQqcjArn2/K8Tlakm+Wxrz7Z7G5 b9F7x1olYOK5jEA1n9cFgOIAfMabSJJjZ0EnCUdEdIeaM9WskzFQ53wmm/AEC9XhWoedYw9bF18dr q3kUicBGQ7+IbKIsfLpf9YIGNk1Nw4lizSkDUAdSu9BzU1qZaZqAHRkYtT2p7X75aDxj1374KFmgH PUgTOT0dDKrZXGbsz8Zpq7qkSVSM0dW9UxcCc41xcULeBKfgHudvyWBfE4Qze7taKyuhbBZ4OIWuQ FuDvIX6lQ==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gtFp7-0000VJ-Hg; Mon, 11 Feb 2019 17:56:53 +0000 Date: Mon, 11 Feb 2019 09:56:53 -0800 From: Matthew Wilcox To: Jan Kara Cc: Linux Upstream , Peter Zijlstra , Chintan Pandya , "hughd@google.com" , "mawilcox@microsoft.com" , "akpm@linux-foundation.org" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: Re: [RFC 1/2] page-flags: Make page lock operation atomic Message-ID: <20190211175653.GE12668@bombadil.infradead.org> References: <20190211125337.16099-1-chintan.pandya@oneplus.com> <20190211125337.16099-2-chintan.pandya@oneplus.com> <20190211134607.GA32511@hirez.programming.kicks-ass.net> <364c7595-14f5-7160-d076-35a14c90375a@oneplus.com> <20190211174846.GM19029@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190211174846.GM19029@quack2.suse.cz> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 11, 2019 at 06:48:46PM +0100, Jan Kara wrote: > On Mon 11-02-19 13:59:24, Linux Upstream wrote: > > > > > >> Signed-off-by: Chintan Pandya > > > > > > NAK. > > > > > > This is bound to regress some stuff. Now agreed that using non-atomic > > > ops is tricky, but many are in places where we 'know' there can't be > > > concurrency. > > > > > > If you can show any single one is wrong, we can fix that one, but we're > > > not going to blanket remove all this just because. > > > > Not quite familiar with below stack but from crash dump, found that this > > was another stack running on some other CPU at the same time which also > > updates page cache lru and manipulate locks. > > > > [84415.344577] [20190123_21:27:50.786264]@1 preempt_count_add+0xdc/0x184 > > [84415.344588] [20190123_21:27:50.786276]@1 workingset_refault+0xdc/0x268 > > [84415.344600] [20190123_21:27:50.786288]@1 add_to_page_cache_lru+0x84/0x11c > > [84415.344612] [20190123_21:27:50.786301]@1 ext4_mpage_readpages+0x178/0x714 > > [84415.344625] [20190123_21:27:50.786313]@1 ext4_readpages+0x50/0x60 > > [84415.344636] [20190123_21:27:50.786324]@1 > > __do_page_cache_readahead+0x16c/0x280 > > [84415.344646] [20190123_21:27:50.786334]@1 filemap_fault+0x41c/0x588 > > [84415.344655] [20190123_21:27:50.786343]@1 ext4_filemap_fault+0x34/0x50 > > [84415.344664] [20190123_21:27:50.786353]@1 __do_fault+0x28/0x88 > > > > Not entirely sure if it's racing with the crashing stack or it's simply > > overrides the the bit set by case 2 (mentioned in 0/2). > > So this is interesting. Looking at __add_to_page_cache_locked() nothing > seems to prevent __SetPageLocked(page) in add_to_page_cache_lru() to get > reordered into __add_to_page_cache_locked() after page is actually added to > the xarray. So that one particular instance might benefit from atomic > SetPageLocked or a barrier somewhere between __SetPageLocked() and the > actual addition of entry into the xarray. There's a write barrier when you add something to the XArray, by virtue of the call to rcu_assign_pointer().