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=-3.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,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 D6FFDC35666 for ; Sat, 22 Feb 2020 17:02:50 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 995D320702 for ; Sat, 22 Feb 2020 17:02:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="cC5CpFYD" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 995D320702 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 356726B000A; Sat, 22 Feb 2020 12:02:50 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 306916B000C; Sat, 22 Feb 2020 12:02:50 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 246F66B000D; Sat, 22 Feb 2020 12:02:50 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0239.hostedemail.com [216.40.44.239]) by kanga.kvack.org (Postfix) with ESMTP id 0D0E56B000A for ; Sat, 22 Feb 2020 12:02:50 -0500 (EST) Received: from smtpin25.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id A996E180AD804 for ; Sat, 22 Feb 2020 17:02:49 +0000 (UTC) X-FDA: 76518382458.25.noise73_8cd4ab2cf7f37 X-HE-Tag: noise73_8cd4ab2cf7f37 X-Filterd-Recvd-Size: 3268 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf10.hostedemail.com (Postfix) with ESMTP for ; Sat, 22 Feb 2020 17:02:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Transfer-Encoding :Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=SY35JLLv+vBVjF2AfSVM5A43C4jm9v1brv6q5yWkRzU=; b=cC5CpFYDWqcMFhwSbiePfLpJKa kwoTL5cYHcxmTKP4DMbY102XnLlQ1/AzXURwUHkNRRfyLgSJvx1zzHEhf3reZhSBiyR3e8zzxZAoe lVTUiNE3CXPjPBQntPEKui5NqJqN+VkYowNZbqQ1iFWjRy61gEIz7SI8cJH/Gf816sEfp7aV5upTJ mQzDU8DQZuG0Cz9vLDeWs37a/PRueidkzItJknwFV3xw3PrAsaobmY+hCTKsyAtHlJnVxOo7NEXsl umj5CTV59yw67b98GLGClqC66/erz7kWsWGm1tYystlInM0vFwwgmk85tI/OwDg0x2lSuGmpjJ9TR qVbURNWA==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j5YAY-0005UU-St; Sat, 22 Feb 2020 17:02:22 +0000 Date: Sat, 22 Feb 2020 09:02:22 -0800 From: Matthew Wilcox To: "Longpeng (Mike)" Cc: Qian Cai , akpm@linux-foundation.org, mike.kravetz@oracle.com, kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, arei.gonglei@huawei.com, weidong.huang@huawei.com, weifuqiang@huawei.com, kvm@vger.kernel.org, linux-mm@kvack.org, Sean Christopherson , stable@vger.kernel.org Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset() Message-ID: <20200222170222.GJ24185@bombadil.infradead.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable 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 Sat, Feb 22, 2020 at 02:33:10PM +0800, Longpeng (Mike) wrote: > =E5=9C=A8 2020/2/22 13:23, Qian Cai =E5=86=99=E9=81=93: > >> On Feb 21, 2020, at 10:34 PM, Longpeng(Mike) = wrote: > >> > >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c > >> index dd8737a..90daf37 100644 > >> --- a/mm/hugetlb.c > >> +++ b/mm/hugetlb.c > >> @@ -4910,28 +4910,30 @@ pte_t *huge_pte_offset(struct mm_struct *mm, > >> { > >> pgd_t *pgd; > >> p4d_t *p4d; > >> - pud_t *pud; > >> - pmd_t *pmd; > >> + pud_t *pud, pud_entry; > >> + pmd_t *pmd, pmd_entry; > >> > >> pgd =3D pgd_offset(mm, addr); > >> - if (!pgd_present(*pgd)) > >> + if (!pgd_present(READ_ONCE(*pgd))) > >> return NULL; > >> p4d =3D p4d_offset(pgd, addr); > >> - if (!p4d_present(*p4d)) > >> + if (!p4d_present(READ_ONCE(*p4d))) > >> return NULL; > >=20 > > What=E2=80=99s the point of READ_ONCE() on those two places? > >=20 > As explained in the commit messages, it's for safe(e.g. avoid the compi= lier > mischief). You can also find the same usage in the ARM64's huge_pte_off= set() in > arch/arm64/mm/hugetlbpage.c I rather agree with Qian; if we need something like READ_ONCE() here, why don't we always need it as part of pgd_present()? It seems like an unnecessary burden for every user.