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 03F2AC47082 for ; Mon, 7 Jun 2021 20:21:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DED4460FF1 for ; Mon, 7 Jun 2021 20:21:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231460AbhFGUXj (ORCPT ); Mon, 7 Jun 2021 16:23:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46846 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230502AbhFGUXh (ORCPT ); Mon, 7 Jun 2021 16:23:37 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 32E3FC061574 for ; Mon, 7 Jun 2021 13:21:46 -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=Z7ZLcUGLD1CuKefmti9g45BjfBhezDkXgDccglbrfSo=; b=cB5nat+gRTIFlVwIsyDFgremu6 RFkiCHi/3xBSfucVb4YN+iM0lGj6BGgHHy8mvidLMVRolJ5BUdzwuUKuJZs0tNCPpt+GZVwH/n02w pkqmRRiuVD316PrOQepdlNah8CGIWYZfKKDf0wQzA25qch3s705qMtTFXYf6YY6l1LWaNvdMDMhEd eKJrjWL2NR0U1dSoBivAWlS7xb1uCJws6Jp8X3tQZ0RWfVJynVHKD9a4bXq0D9GQbZ1VvT3mwIcZW 8MsugKcw5YDzyyqFzQ7caCj8/OBtmichSa44yEbKlvVkerijgwJ/WHb/e948a8ruS+G+0id38Ly6P QHKsPWEw==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lqLjn-00GCqZ-G7; Mon, 07 Jun 2021 20:20:53 +0000 Date: Mon, 7 Jun 2021 21:20:43 +0100 From: Matthew Wilcox To: Jann Horn Cc: Linux-MM , Zi Yan , Peter Xu , "Kirill A. Shutemov" , Konstantin Khlebnikov , Andrew Morton , chinwen.chang@mediatek.com, kernel list , syzkaller-bugs , Vlastimil Babka , Michel Lespinasse , syzbot Subject: Re: split_huge_page_to_list() races with page_mapcount() on migration entry in smaps code? [was: Re: [syzbot] kernel BUG in __page_mapcount] Message-ID: References: <00000000000017977605c395a751@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 07, 2021 at 09:55:09PM +0200, Jann Horn wrote: > On Mon, Jun 7, 2021 at 8:03 PM Matthew Wilcox wrote: > > On Mon, Jun 07, 2021 at 07:27:23PM +0200, Jann Horn wrote: > > > === Short summary === > > > I believe the issue here is a race between /proc/*/smaps and > > > split_huge_page_to_list(): > > > > > > The codepath for /proc/*/smaps walks the pagetables and (e.g. in > > > smaps_account()) calls page_mapcount() not just on pages from normal > > > PTEs but also on migration entries (since commit b1d4d9e0cbd0a > > > "proc/smaps: carefully handle migration entries", from Linux v3.5). > > > page_mapcount() expects compound pages to be stable. > > > > > > The split_huge_page_to_list() path first protects the compound page by > > > locking it and replacing all its PTEs with migration entries (since > > > the THP rewrite in v4.5, I think?), then does the actual splitting > > > using __split_huge_page(). > > > > > > So there's a mismatch of expectations here: > > > The smaps code expects that migration entries point to stable compound > > > pages, while the THP code expects that it's okay to split a compound > > > page while it has migration entries. > > > > Will it be a colossal performance penalty if we always get the page > > refcount after looking it up? That will cause split_huge_page() to > > fail to split the page if it hits this race. > > Hmm - but with that approach I'm not sure you could even easily take a > refcount on a page whose refcount may be frozen and which may be in > the middle of being shattered? get_page_unless_zero() is wrong because > you can't take references on tail pages, right? (Or can you?) And > try_get_page() is wrong because it bugs out if the refcount is zero - > and even if it didn't do that, you might end up holding a reference on > the head page while the page you're actually interested in is a tail > page? > > I guess if it was really necessary, it'd be possible to do some kind > of retry thing that grabs a reference on the compound head, then > checks that the tail page is still associated with the compound head, > and if not, drops the compound head and tries again? Right; that's how get_user_page_fast() works -- see try_get_compound_head(). If it can't get the reference, it just fails. I suspect for smaps, we can just choose to not count the page. It'll be an inaccuracy in the stats, but I don't think that's a big deal.