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 BC7BAC49ED7 for ; Thu, 19 Sep 2019 15:41:53 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 5F8E721907 for ; Thu, 19 Sep 2019 15:41:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5F8E721907 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id BAE756B0381; Thu, 19 Sep 2019 11:41:52 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B35406B0383; Thu, 19 Sep 2019 11:41:52 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9FC5D6B0384; Thu, 19 Sep 2019 11:41:52 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0182.hostedemail.com [216.40.44.182]) by kanga.kvack.org (Postfix) with ESMTP id 77A966B0381 for ; Thu, 19 Sep 2019 11:41:52 -0400 (EDT) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 30C356D91 for ; Thu, 19 Sep 2019 15:41:52 +0000 (UTC) X-FDA: 75952085664.15.hen21_6749558b1225c X-HE-Tag: hen21_6749558b1225c X-Filterd-Recvd-Size: 5122 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf45.hostedemail.com (Postfix) with ESMTP for ; Thu, 19 Sep 2019 15:41:49 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id B199628; Thu, 19 Sep 2019 08:41:48 -0700 (PDT) Received: from arrakis.emea.arm.com (arrakis.cambridge.arm.com [10.1.196.78]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E65BC3F575; Thu, 19 Sep 2019 08:41:45 -0700 (PDT) Date: Thu, 19 Sep 2019 16:41:43 +0100 From: Catalin Marinas To: "Kirill A. Shutemov" Cc: Jia He , Will Deacon , Mark Rutland , James Morse , Marc Zyngier , Matthew Wilcox , "Kirill A. Shutemov" , 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: <20190919154143.GA6472@arrakis.emea.arm.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190919150007.k7scjplcya53j7r4@box> User-Agent: Mutt/1.10.1 (2018-07-13) 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 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. 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. -- Catalin