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=-12.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 9DFCDC47082 for ; Wed, 26 May 2021 21:46:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 71E05600CD for ; Wed, 26 May 2021 21:46:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234127AbhEZVsK (ORCPT ); Wed, 26 May 2021 17:48:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48126 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232976AbhEZVsI (ORCPT ); Wed, 26 May 2021 17:48:08 -0400 Received: from mail-ed1-x536.google.com (mail-ed1-x536.google.com [IPv6:2a00:1450:4864:20::536]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 694F9C061574 for ; Wed, 26 May 2021 14:46:36 -0700 (PDT) Received: by mail-ed1-x536.google.com with SMTP id h16so3285980edr.6 for ; Wed, 26 May 2021 14:46:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=TVadEvLZpce8a7tGz4LRC6JJpJnbOhzGVc5VcId76RQ=; b=F6o4xrQad6efMACHqorToK30W9yh7Gf5y2FY3ZphmDLLWR1oRR+ukXzN+SBzCctnpg 8PIU+XYUi8kMTij/+vo1hqtIjzjSg+MVC4qMobaMH50z/gL+Aamxq7VUqS2M/3RmDoUW FBM4mZJXA9Lw1hr6BMbSwexgGedX4fRFUG60qaHj8OSIJFCXx4KTRYbMJw8in5gSeg+F wXKsf09T1i1/zZ68WG5NEYezvhsHQFdVDnajcI2gZz8Qo9vZv8f3D+efFGw0wxhfs5k5 56lQp+rpZAsi88fVFP+/s6380xa5tZhuzuJu+ppcYIWGjrvn3IShNRPTR7Z3o2n4gL8P c/mw== 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=TVadEvLZpce8a7tGz4LRC6JJpJnbOhzGVc5VcId76RQ=; b=ZZG8/DPjGo6xjKKW2SwtQ1sgjeyjfRgXsbEx5UHVMPlWtsnmxI9GBY0FgezlI2aGxb LFExoMP7I4lD7sBk7VEFGM9Qpy62IMqXew33AUNnuwlLdwsOh6hCJJ5ZYAKvLlNW7zNr Tr8S9PZeEkFg6f2KheuAIRIkyzK02XR+tidS3IQoVat+3H4q7zATzkLTh+LMvAy3RKa/ TG13NlN6ntSYGmil6XaULrXrTwytgqOZDFbtKdNEBiOM4wkUx16Kv0UYdljk75bG6BSo 3XtqzlNoXm13yCVLo72Vg1WfDoyNniN0eL2HW7SwjQePbSDc1+u0TYxREsBglZYliNOC yAcQ== X-Gm-Message-State: AOAM533SwSJ0QCE4azdKoELVWNFWfBJpuHAvYLz7xgKnxn5VeonvuD9f /vyaM/Gcgr4jOSqjgRYEedU82auU39k42SmB7KM= X-Google-Smtp-Source: ABdhPJwNv31/gORIrWNfhwq738PIQ709IiWGpudsg02wgTgLJy5kALrFpTNSNH/CXJIznBUzJY9JraVh+chdmt0bpao= X-Received: by 2002:a05:6402:3551:: with SMTP id f17mr277738edd.313.1622065594992; Wed, 26 May 2021 14:46:34 -0700 (PDT) MIME-Version: 1.0 References: <20210525162145.3510-1-shy828301@gmail.com> <20210525162145.3510-2-shy828301@gmail.com> In-Reply-To: From: Yang Shi Date: Wed, 26 May 2021 14:46:23 -0700 Message-ID: Subject: Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split To: Hugh Dickins Cc: Zi Yan , "Kirill A. Shutemov" , =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= , Wang Yugui , Andrew Morton , Linux MM , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 25, 2021 at 4:58 PM Hugh Dickins wrote: > > On Tue, 25 May 2021, Yang Shi wrote: > > On Tue, May 25, 2021 at 3:06 PM Hugh Dickins wrote: > > > On Tue, 25 May 2021, Yang Shi wrote: > > > > > > > When debugging the bug reported by Wang Yugui [1], try_to_unmap() may > > > > return false positive for PTE-mapped THP since page_mapcount() is used > > > > to check if the THP is unmapped, but it just checks compound mapount and > > > > head page's mapcount. If the THP is PTE-mapped and head page is not > > > > mapped, it may return false positive. > > > > > > But those false positives did not matter because there was a separate > > > DEBUG_VM check later. > > > > > > It's good to have the link to Wang Yugui's report, but that paragraph > > > is not really about this patch, as it has evolved now: this patch > > > consolidates the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE. > > > > > > > > > > > The try_to_unmap() has been changed to void function, so check > > > > page_mapped() after it. And changed BUG_ON to WARN_ON since it is not a > > > > fatal issue. > > > > > > The change from DEBUG_VM BUG to VM_WARN_ON_ONCE is the most important > > > part of this, and the reason it's good for stable: and the patch title > > > ought to highlight that, not the page_mapcount business. > > > > Will update the subject and the commit log accordingly. > > Thanks! > > > > > > > > > > > > > > [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/ > > > > > > > > Reviewed-by: Zi Yan > > > > Signed-off-by: Yang Shi > > > > > > This will be required Cc: stable@vger.kernel.org > > > (but we don't want to Cc them on this mail). > > > > > > As I said on the other, I think this should be 1/2 not 2/2. > > > > Sure. > > Great. > > > > > > > > > > --- > > > > v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag > > > > since there is no fundamental change against v2. > > > > v2: Removed dead code and updated the comment of try_to_unmap() per Zi > > > > Yan. > > > > mm/huge_memory.c | 17 +++++------------ > > > > 1 file changed, 5 insertions(+), 12 deletions(-) > > > > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > > index 80fe642d742d..72d81d8e01b1 100644 > > > > --- a/mm/huge_memory.c > > > > +++ b/mm/huge_memory.c > > > > @@ -2343,6 +2343,8 @@ static void unmap_page(struct page *page) > > > > ttu_flags |= TTU_SPLIT_FREEZE; > > > > > > > > try_to_unmap(page, ttu_flags); > > > > + > > > > + VM_WARN_ON_ONCE_PAGE(page_mapped(page), page); > > > > > > There is one useful piece of information that dump_page() will not show: > > > total_mapcount(page). Is there a way of crafting that into the output? > > > > > > Not with the macros available, I think. Maybe we should be optimistic > > > and assume I already have the fixes, so not worth trying to refine the > > > message (but I'm not entirely convinced of that!). > > > > > > The trouble with > > > if (VM_WARN_ON_ONCE_PAGE(page_mapped(page), page)) > > > pr_warn("total_mapcount:%d\n", total_mapcount(page)); > > > is that it's printed regardless of the ONCEness. Another "trouble" > > > is that it's printed so long after the page_mapped(page) check that > > > it may be 0 by now - but one can see that as itself informative. > > > > We should be able to make dump_page() print total mapcount, right? The > > dump_page() should be just called in some error paths so taking some > > extra overhead to dump more information seems harmless, or am I > > missing something? Of course, this can be done in a separate patch. > > I didn't want to ask that of you, but yes, if you're willing to add > total_mapcount() into dump_page(), I think that would be ideal; and > could be helpful for other cases too. > > Looking through total_mapcount(), I think it's safe to call from > dump_page() - I always worry about extending crash info with > something that depends on a maybe-corrupted pointer which would > generate a further crash and either recurse or truncate the output - > but please check that carefully. Yes, it is possible. If the THP is being split, some VM_BUG_* might be triggered if total_mapcount() is called. But it is still feasible to print total mapcount as long as we implement a more robust version for dump_page(). > > Yes, a separate patch please: which can come later on, and no > need for stable for that one, but good to know it's coming. > > Thanks, > Hugh