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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 3C0FEC49ED7 for ; Thu, 19 Sep 2019 15:51:36 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id EC2AD208C0 for ; Thu, 19 Sep 2019 15:51:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EC2AD208C0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 6EB876B0384; Thu, 19 Sep 2019 11:51:35 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 69A0F6B0385; Thu, 19 Sep 2019 11:51:35 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5B0B36B0386; Thu, 19 Sep 2019 11:51:35 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0189.hostedemail.com [216.40.44.189]) by kanga.kvack.org (Postfix) with ESMTP id 3993B6B0384 for ; Thu, 19 Sep 2019 11:51:35 -0400 (EDT) Received: from smtpin11.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id D4BB855F8E for ; Thu, 19 Sep 2019 15:51:34 +0000 (UTC) X-FDA: 75952110108.11.grip59_2ac91ecdfe95d X-HE-Tag: grip59_2ac91ecdfe95d X-Filterd-Recvd-Size: 5886 Received: from mga05.intel.com (mga05.intel.com [192.55.52.43]) by imf13.hostedemail.com (Postfix) with ESMTP for ; Thu, 19 Sep 2019 15:51:33 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by fmsmga105.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 19 Sep 2019 08:51:32 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,524,1559545200"; d="scan'208";a="178091248" Received: from black.fi.intel.com ([10.237.72.28]) by orsmga007.jf.intel.com with ESMTP; 19 Sep 2019 08:51:25 -0700 Received: by black.fi.intel.com (Postfix, from userid 1000) id 61703BD; Thu, 19 Sep 2019 18:51:24 +0300 (EEST) Date: Thu, 19 Sep 2019 18:51:24 +0300 From: "Kirill A. Shutemov" To: Catalin Marinas Cc: "Kirill A. Shutemov" , Jia He , Will Deacon , Mark Rutland , James Morse , Marc Zyngier , Matthew Wilcox , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Suzuki Poulose , Punit Agrawal , Anshuman Khandual , Jun Yao , Alex Van Brunt , Robin Murphy , Thomas Gleixner , Andrew Morton , =?iso-8859-1?B?Suly9G1l?= Glisse , Ralph Campbell , hejianet@gmail.com, Kaly Xin Subject: Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared Message-ID: <20190919155124.56ps5vsd5al6g7hm@black.fi.intel.com> References:<20190918131914.38081-1-justin.he@arm.com> <20190918131914.38081-4-justin.he@arm.com> <20190918140027.ckj32xnryyyesc23@box> <20190918180029.GB20601@iMac.local> <20190919150007.k7scjplcya53j7r4@box> <20190919154143.GA6472@arrakis.emea.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To:<20190919154143.GA6472@arrakis.emea.arm.com> User-Agent: NeoMutt/20170714-126-deb55f (1.8.3) 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 Thu, Sep 19, 2019 at 03:41:43PM +0000, Catalin Marinas wrote: > On Thu, Sep 19, 2019 at 06:00:07PM +0300, Kirill A. Shutemov wrote: > > On Wed, Sep 18, 2019 at 07:00:30PM +0100, Catalin Marinas wrote: > > > On Wed, Sep 18, 2019 at 05:00:27PM +0300, Kirill A. Shutemov wrote: > > > > On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote: > > > > > @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo > > > > > */ > > > > > if (unlikely(!src)) { > > > > > void *kaddr = kmap_atomic(dst); > > > > > - void __user *uaddr = (void __user *)(va & PAGE_MASK); > > > > > + void __user *uaddr = (void __user *)(addr & PAGE_MASK); > > > > > + pte_t entry; > > > > > > > > > > /* > > > > > * This really shouldn't fail, because the page is there > > > > > * in the page tables. But it might just be unreadable, > > > > > * in which case we just give up and fill the result with > > > > > - * zeroes. > > > > > + * zeroes. On architectures with software "accessed" bits, > > > > > + * we would take a double page fault here, so mark it > > > > > + * accessed here. > > > > > */ > > > > > + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) { > > > > > + spin_lock(vmf->ptl); > > > > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) { > > > > > + entry = pte_mkyoung(vmf->orig_pte); > > > > > + if (ptep_set_access_flags(vma, addr, > > > > > + vmf->pte, entry, 0)) > > > > > + update_mmu_cache(vma, addr, vmf->pte); > > > > > + } > > > > > > > > I don't follow. > > > > > > > > So if pte has changed under you, you don't set the accessed bit, but never > > > > the less copy from the user. > > > > > > > > What makes you think it will not trigger the same problem? > > > > > > > > I think we need to make cow_user_page() fail in this case and caller -- > > > > wp_page_copy() -- return zero. If the fault was solved by other thread, we > > > > are fine. If not userspace would re-fault on the same address and we will > > > > handle the fault from the second attempt. > > > > > > It would be nice to clarify the semantics of this function and do as > > > you suggest but the current comment is slightly confusing: > > > > > > /* > > > * If the source page was a PFN mapping, we don't have > > > * a "struct page" for it. We do a best-effort copy by > > > * just copying from the original user address. If that > > > * fails, we just zero-fill it. Live with it. > > > */ > > > > > > Would any user-space rely on getting a zero-filled page here instead of > > > a recursive fault? > > > > I don't see the point in zero-filled page in this case. SIGBUS sounds like > > more appropriate response, no? > > I think misunderstood your comment. So, if !pte_same(), we should let > userspace re-fault. This wouldn't be a user ABI change and it is > bounded, can't end up in an infinite re-fault loop. Right. !pte_same() can only happen if we raced with somebody else. I cannot imagine situation when the race will happen more than few times in a row. > In case of a __copy_from_user_inatomic() error, SIGBUS would make more > sense but it changes the current behaviour (zero-filling the page). This > can be left for a separate patch, doesn't affect the arm64 case here. I think it's safer to leave it as is. Maybe put WARN_ON_ONCE() or something. There can be some obscure use-case that I don't see. -- Kirill A. Shutemov