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=-3.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 6D7C9C433E0 for ; Fri, 8 Jan 2021 02:39:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2EEDB23603 for ; Fri, 8 Jan 2021 02:39:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727171AbhAHCjc (ORCPT ); Thu, 7 Jan 2021 21:39:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57888 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726113AbhAHCjb (ORCPT ); Thu, 7 Jan 2021 21:39:31 -0500 Received: from mail-pl1-x62a.google.com (mail-pl1-x62a.google.com [IPv6:2607:f8b0:4864:20::62a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 735D0C0612F5 for ; Thu, 7 Jan 2021 18:38:51 -0800 (PST) Received: by mail-pl1-x62a.google.com with SMTP id r4so4889163pls.11 for ; Thu, 07 Jan 2021 18:38:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=4zmgQQ/koFoyCFqZqHd/CEfBclNLFL5+ci7EJGrdRVA=; b=F8LuEnnlkXA62BJVlXrdGxp+p35rEVAysJ0KXywPGVixsnKoZorWYmTHSbzjyEX1cu 6JReREQh9aZWC+YPkux2w9e2uZqA1Mi8djI2bKF2/6mZAxXMFQeJVW9Aw+tgo8BjFEPd aFksKmnjXlqAF0JkywhAJyuNgkQLXeFYSHIt9ngMAlugUyIcquP5wTTjGKZx7fmEZk5N YvBhdV8dEi/eR/cn+15+0raiD4KeFSKOenp75hpfLoMBtNwwXmsMBAP4Kg5CNQp7PHsK 4GKNpv5yEZafGYiqyCRB7avkZ9mNmtff+ajJ6IN6/IPx0Ux6S3hqyRGieqqUNNyVVEm5 AEGw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=4zmgQQ/koFoyCFqZqHd/CEfBclNLFL5+ci7EJGrdRVA=; b=FYnKkcQnmv7Yl+tVCxOhB+vfpnaVdGBFQgoC9vN9DTBJ7UrSwLM6nUzmFIRelEBmZW gxznvCp0L88qe9WkeLmEpQrp3GKKD3OC4RIc2eQ7//BnRcexT/e+63LJb4bb3JBBJy9/ OCFLYkjbgKO0vdm8Z90u9UgF6kLKLlIiK9OSiVrXgX8V015ns7/EAwxR58eGwI1XP+IF RZf+oOMdpLobTPYAb8dglCSBr/l0B29IRlUOaZjdCNKaKCdnP799i4XaUnc+4Rcu5qhx xvWeXMLNW7LGzc4e9PxRvN9y1p921I8lFHfD14zA1nXb1K9oAEMpGHvEn0rIgLBSE7o4 Wr7A== X-Gm-Message-State: AOAM533oBtb7kLoT2dCI0kTJ1kJdH9reJG80NLf3saYLUONiLM0f/xww t66j78VlolkFK5G8gao8USkLzQWMUkBN8WmOGdBs2w== X-Google-Smtp-Source: ABdhPJys/3J9TCNs5W2zE23cKCcRfCrUXDSYSMltBveYwqJe9Yzwi+mY1FkbaxCByDOgONhDCRFnZMzB1Ndm2rNdMbw= X-Received: by 2002:a17:90a:c588:: with SMTP id l8mr1414168pjt.147.1610073531034; Thu, 07 Jan 2021 18:38:51 -0800 (PST) MIME-Version: 1.0 References: <20210106084739.63318-1-songmuchun@bytedance.com> <20210106084739.63318-4-songmuchun@bytedance.com> <20210106165632.GT13207@dhcp22.suse.cz> <20210107084146.GD13207@dhcp22.suse.cz> <20210107111827.GG13207@dhcp22.suse.cz> <20210107123854.GJ13207@dhcp22.suse.cz> <20210107141130.GL13207@dhcp22.suse.cz> In-Reply-To: From: Muchun Song Date: Fri, 8 Jan 2021 10:38:12 +0800 Message-ID: Subject: Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page To: Mike Kravetz Cc: Michal Hocko , Andrew Morton , Naoya Horiguchi , Andi Kleen , Linux Memory Management List , LKML Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 8, 2021 at 9:08 AM Mike Kravetz wrote: > > On 1/7/21 7:11 AM, Muchun Song wrote: > > On Thu, Jan 7, 2021 at 10:11 PM Michal Hocko wrote: > >> > >> On Thu 07-01-21 20:59:33, Muchun Song wrote: > >>> On Thu, Jan 7, 2021 at 8:38 PM Michal Hocko wrote: > >> [...] > >>>> Right. Can we simply back off in the dissolving path when ref count is > >>>> 0 && PageHuge() if list_empty(page->lru)? Is there any other scenario > >>>> when the all above is true and the page is not being freed? > >>> > >>> The list_empty(&page->lru) may always return false. > >>> The page before freeing is on the active list > >>> (hstate->hugepage_activelist).Then it is on the free list > >>> after freeing. So list_empty(&page->lru) is always false. > >> > >> The point I was trying to make is that the page has to be enqueued when > >> it is dissolved and freed. If the page is not enqueued then something > >> racing. But then I have realized that this is not a great check to > >> detect the race because pages are going to be released to buddy > >> allocator and that will reuse page->lru again. So scratch that and sorry > >> for the detour. > >> > >> But that made me think some more and one way to reliably detect the race > >> should be PageHuge() check in the freeing path. This is what dissolve > >> path does already. PageHuge becomes false during update_and_free_page() > >> while holding the hugetlb_lock. So can we use that? > > > > It may make the thing complex. Apart from freeing it to the > > buddy allocator, free_huge_page also does something else for > > us. If we detect the race in the freeing path, if it is not a HugeTLB > > page, the freeing path just returns. We also should move those > > things to the dissolve path. Right? > > > > But I find a tricky problem to solve. See free_huge_page(). > > If we are in non-task context, we should schedule a work > > to free the page. We reuse the page->mapping. If the page > > is already freed by the dissolve path. We should not touch > > the page->mapping. So we need to check PageHuge(). > > The check and llist_add() should be protected by > > hugetlb_lock. But we cannot do that. Right? If dissolve > > happens after it is linked to the list. We also should > > remove it from the list (hpage_freelist). It seems to make > > the thing more complex. > > You are correct. This is also an issue/potential problem with this > race. It seems that adding the state information might be the least > complex way to address issue. Yeah, I agree with you. Adding a state is a simple solution. > > -- > Mike Kravetz