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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 EAEFDC433FF for ; Thu, 8 Aug 2019 07:46:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C0DF82171F for ; Thu, 8 Aug 2019 07:46:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1565250371; bh=KQGFnO+gq1uo790n4QAS3mxbsxvibaSyNvsjuI9WHBM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=ziExlXcRvKEnUdyFHNjeKYpCWaEt2yXpdDCRj06H96TWYbcNZWZahr2vEHYAHrIjk gsI1PZVneZFDTVCtrpslweRsuyJH1w1j0wzFSrEPRBKKrPEWB14lQ0dS1FT675yD14 q0Av04W1VsJEkVF8a/Yb3o9y0FBVyHdp3N3AtAQQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731454AbfHHHqK (ORCPT ); Thu, 8 Aug 2019 03:46:10 -0400 Received: from mx2.suse.de ([195.135.220.15]:56828 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730887AbfHHHqK (ORCPT ); Thu, 8 Aug 2019 03:46:10 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 36F83B11C; Thu, 8 Aug 2019 07:46:08 +0000 (UTC) Date: Thu, 8 Aug 2019 09:46:07 +0200 From: Michal Hocko To: Mike Kravetz Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, ltp@lists.linux.it, Li Wang , Naoya Horiguchi , Cyril Hrubis , xishi.qiuxishi@alibaba-inc.com, Andrew Morton Subject: Re: [PATCH] hugetlbfs: fix hugetlb page migration/fault race causing SIGBUS Message-ID: <20190808074607.GI11812@dhcp22.suse.cz> References: <20190808000533.7701-1-mike.kravetz@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190808000533.7701-1-mike.kravetz@oracle.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 07-08-19 17:05:33, Mike Kravetz wrote: > Li Wang discovered that LTP/move_page12 V2 sometimes triggers SIGBUS > in the kernel-v5.2.3 testing. This is caused by a race between hugetlb > page migration and page fault. > > If a hugetlb page can not be allocated to satisfy a page fault, the task > is sent SIGBUS. This is normal hugetlbfs behavior. A hugetlb fault > mutex exists to prevent two tasks from trying to instantiate the same > page. This protects against the situation where there is only one > hugetlb page, and both tasks would try to allocate. Without the mutex, > one would fail and SIGBUS even though the other fault would be successful. > > There is a similar race between hugetlb page migration and fault. > Migration code will allocate a page for the target of the migration. > It will then unmap the original page from all page tables. It does > this unmap by first clearing the pte and then writing a migration > entry. The page table lock is held for the duration of this clear and > write operation. However, the beginnings of the hugetlb page fault > code optimistically checks the pte without taking the page table lock. > If clear (as it can be during the migration unmap operation), a hugetlb > page allocation is attempted to satisfy the fault. Note that the page > which will eventually satisfy this fault was already allocated by the > migration code. However, the allocation within the fault path could > fail which would result in the task incorrectly being sent SIGBUS. > > Ideally, we could take the hugetlb fault mutex in the migration code > when modifying the page tables. However, locks must be taken in the > order of hugetlb fault mutex, page lock, page table lock. This would > require significant rework of the migration code. Instead, the issue > is addressed in the hugetlb fault code. After failing to allocate a > huge page, take the page table lock and check for huge_pte_none before > returning an error. This is the same check that must be made further > in the code even if page allocation is successful. > > Reported-by: Li Wang > Fixes: 290408d4a250 ("hugetlb: hugepage migration core") > Signed-off-by: Mike Kravetz > Tested-by: Li Wang Acked-by: Michal Hocko Thanks! > --- > mm/hugetlb.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index ede7e7f5d1ab..6d7296dd11b8 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -3856,6 +3856,25 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm, > > page = alloc_huge_page(vma, haddr, 0); > if (IS_ERR(page)) { > + /* > + * Returning error will result in faulting task being > + * sent SIGBUS. The hugetlb fault mutex prevents two > + * tasks from racing to fault in the same page which > + * could result in false unable to allocate errors. > + * Page migration does not take the fault mutex, but > + * does a clear then write of pte's under page table > + * lock. Page fault code could race with migration, > + * notice the clear pte and try to allocate a page > + * here. Before returning error, get ptl and make > + * sure there really is no pte entry. > + */ > + ptl = huge_pte_lock(h, mm, ptep); > + if (!huge_pte_none(huge_ptep_get(ptep))) { > + ret = 0; > + spin_unlock(ptl); > + goto out; > + } > + spin_unlock(ptl); > ret = vmf_error(PTR_ERR(page)); > goto out; > } > -- > 2.20.1 -- Michal Hocko SUSE Labs