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=-24.8 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL 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 BAA9EC4707F for ; Tue, 25 May 2021 22:06:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8E7A661402 for ; Tue, 25 May 2021 22:06:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231410AbhEYWHb (ORCPT ); Tue, 25 May 2021 18:07:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37832 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230097AbhEYWHb (ORCPT ); Tue, 25 May 2021 18:07:31 -0400 Received: from mail-oi1-x22b.google.com (mail-oi1-x22b.google.com [IPv6:2607:f8b0:4864:20::22b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2E784C061574 for ; Tue, 25 May 2021 15:06:01 -0700 (PDT) Received: by mail-oi1-x22b.google.com with SMTP id v22so31769736oic.2 for ; Tue, 25 May 2021 15:06:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:in-reply-to:message-id:references :user-agent:mime-version; bh=V2f60hFSBg5mplGHJu5ZXvvWvgziSj8114cWWDTA9tc=; b=VeBJ+qroJZ6lptIFw3vuc9CAr1u2l2n5YGdEIfOeOH4SCoiF55HE9g4PD3/QE1mqrN bI9EsboT3lIAjhVvshAIfIGN6cNgA7rz5wRSQtn0eoP3N7W4i4E3vtXo6ufOFWuDXIpX Fl5oe0SdmGhrC8QKe0wgmnDAWMtpXqH/5I4YaWqVguwcB0+X2eFPmutCzbU++NoS91sB XClFoObDbjFLvtZ11xAffuoIU+Ws04AWl1y7WfRtN/SmK/TG8LzfmK9ooXsNDSGuIQhi LKUae758Zkf4z2ijuw7h6QAXM5JLW7PummJH1sqSmx9kIAdh32xA/Si/KsHmiYcMcMS+ oqiQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version; bh=V2f60hFSBg5mplGHJu5ZXvvWvgziSj8114cWWDTA9tc=; b=CZmZAW6VG4a9AREbSS0exH8CP/dCm5jB9sktuDxqVvEnRdLADaCALrWRlxramu5ORP 6ncC4tMq3Xi71xWbfm05szcH6gGhU2OUMK48yC96ddwHeVtecoiigo2jxg8DI7ByR5x7 tl2sdTJAk6BZ+CGTP0RXirD4RN+FHhWFWvtLDvCYGKNA/aBAnYodsRdDNaH2PvzZ2b2e JNE7fML1iL47K+3MG86cqTYn7/m+KwCdTylGZj7VZl5AFs/ZUOkB1xbtVtqlGjqHQLo3 oiYvhahgzMPCV9f9XoXOijnl4JbwXzx4c2un8L8B42Q7xEXmHgLg9QxsGeEa04uyMZjn vUgA== X-Gm-Message-State: AOAM530whCrg8LbBRKZ9BHso7s+HNKiyxubo64dkgNdt7qlI5/wKQtf6 8SbzO52pUnULBt9yN5bTP+EqVw== X-Google-Smtp-Source: ABdhPJy1tKFmLZf9y03mRZLZ33XkSMEIxr0p6rAiykzoBjdZLKDgJFm9fANgsdRfPpIXyTD9WBUh1A== X-Received: by 2002:aca:cf8d:: with SMTP id f135mr309891oig.33.1621980360190; Tue, 25 May 2021 15:06:00 -0700 (PDT) Received: from eggly.attlocal.net (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id v19sm4052931otq.35.2021.05.25.15.05.59 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Tue, 25 May 2021 15:05:59 -0700 (PDT) Date: Tue, 25 May 2021 15:05:57 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Yang Shi cc: hughd@google.com, ziy@nvidia.com, kirill.shutemov@linux.intel.com, naoya.horiguchi@nec.com, wangyugui@e16-tech.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split In-Reply-To: <20210525162145.3510-2-shy828301@gmail.com> Message-ID: References: <20210525162145.3510-1-shy828301@gmail.com> <20210525162145.3510-2-shy828301@gmail.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > [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. > --- > 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. I guess leave it as you have it, I don't see an easy better (without going back to something like the old contortions). > } > > static void remap_page(struct page *page, unsigned int nr) > @@ -2653,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > struct deferred_split *ds_queue = get_deferred_split_queue(head); > struct anon_vma *anon_vma = NULL; > struct address_space *mapping = NULL; > - int count, mapcount, extra_pins, ret; > + int mapcount, extra_pins, ret; Remove mapcount too. > pgoff_t end; > > VM_BUG_ON_PAGE(is_huge_zero_page(head), head); > @@ -2712,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > } > > unmap_page(head); > - VM_BUG_ON_PAGE(compound_mapcount(head), head); > > /* block interrupt reentry in xa_lock and spinlock */ > local_irq_disable(); > @@ -2730,7 +2731,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > > /* Prevent deferred_split_scan() touching ->_refcount */ > spin_lock(&ds_queue->split_queue_lock); > - count = page_count(head); > mapcount = total_mapcount(head); > if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) { mapcount was useful for printing in the hand-crafted message deleted, but serves no purpose now: just do the page_ref_freeze() without it. > if (!list_empty(page_deferred_list(head))) { > @@ -2752,16 +2752,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > __split_huge_page(page, list, end); > ret = 0; > } else { > - if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) { > - pr_alert("total_mapcount: %u, page_count(): %u\n", > - mapcount, count); > - if (PageTail(page)) > - dump_page(head, NULL); > - dump_page(page, "total_mapcount(head) > 0"); > - BUG(); > - } > spin_unlock(&ds_queue->split_queue_lock); > -fail: if (mapping) > +fail: > + if (mapping) > xa_unlock(&mapping->i_pages); > local_irq_enable(); > remap_page(head, thp_nr_pages(head)); > -- > 2.26.2 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=-24.8 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL autolearn=unavailable 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 9A539C47086 for ; 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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id v19sm4052931otq.35.2021.05.25.15.05.59 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Tue, 25 May 2021 15:05:59 -0700 (PDT) Date: Tue, 25 May 2021 15:05:57 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Yang Shi cc: hughd@google.com, ziy@nvidia.com, kirill.shutemov@linux.intel.com, naoya.horiguchi@nec.com, wangyugui@e16-tech.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split In-Reply-To: <20210525162145.3510-2-shy828301@gmail.com> Message-ID: References: <20210525162145.3510-1-shy828301@gmail.com> <20210525162145.3510-2-shy828301@gmail.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=VeBJ+qro; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf27.hostedemail.com: domain of hughd@google.com designates 209.85.167.178 as permitted sender) smtp.mailfrom=hughd@google.com X-Stat-Signature: jtyxrn5xrxekxn6sngnq8db6y3qyf7hn X-Rspamd-Queue-Id: A609D8019120 X-Rspamd-Server: rspam02 X-HE-Tag: 1621980354-455672 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. > > [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. > --- > 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. I guess leave it as you have it, I don't see an easy better (without going back to something like the old contortions). > } > > static void remap_page(struct page *page, unsigned int nr) > @@ -2653,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > struct deferred_split *ds_queue = get_deferred_split_queue(head); > struct anon_vma *anon_vma = NULL; > struct address_space *mapping = NULL; > - int count, mapcount, extra_pins, ret; > + int mapcount, extra_pins, ret; Remove mapcount too. > pgoff_t end; > > VM_BUG_ON_PAGE(is_huge_zero_page(head), head); > @@ -2712,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > } > > unmap_page(head); > - VM_BUG_ON_PAGE(compound_mapcount(head), head); > > /* block interrupt reentry in xa_lock and spinlock */ > local_irq_disable(); > @@ -2730,7 +2731,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > > /* Prevent deferred_split_scan() touching ->_refcount */ > spin_lock(&ds_queue->split_queue_lock); > - count = page_count(head); > mapcount = total_mapcount(head); > if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) { mapcount was useful for printing in the hand-crafted message deleted, but serves no purpose now: just do the page_ref_freeze() without it. > if (!list_empty(page_deferred_list(head))) { > @@ -2752,16 +2752,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) > __split_huge_page(page, list, end); > ret = 0; > } else { > - if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) { > - pr_alert("total_mapcount: %u, page_count(): %u\n", > - mapcount, count); > - if (PageTail(page)) > - dump_page(head, NULL); > - dump_page(page, "total_mapcount(head) > 0"); > - BUG(); > - } > spin_unlock(&ds_queue->split_queue_lock); > -fail: if (mapping) > +fail: > + if (mapping) > xa_unlock(&mapping->i_pages); > local_irq_enable(); > remap_page(head, thp_nr_pages(head)); > -- > 2.26.2