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[23.128.96.18]) by mx.google.com with ESMTP id f12si544597edq.119.2020.04.30.15.22.03; Thu, 30 Apr 2020 15:22:27 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=intel.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727068AbgD3WUJ (ORCPT + 99 others); Thu, 30 Apr 2020 18:20:09 -0400 Received: from mga06.intel.com ([134.134.136.31]:64761 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726447AbgD3WUJ (ORCPT ); Thu, 30 Apr 2020 18:20:09 -0400 IronPort-SDR: Nmy2SMonhEF9N5yObYCShGyE0JIviHsvavAck0uweUqq+6nFtjb8JOeBJhhTv2evzjQMA3RNZx 1FePlNckYeAg== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga006.jf.intel.com ([10.7.209.51]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Apr 2020 15:20:08 -0700 IronPort-SDR: T7uBwXiNPRoS+3hMz5KdpH/1eTYmxcBu2tj7GmZEFouoQWuhYFf2wiVPVtTggMbBWOoPKAxDeG ZOTJnFvd6Opw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,337,1583222400"; d="scan'208";a="261929251" Received: from dnlloyd-mobl.amr.corp.intel.com (HELO [10.255.231.251]) ([10.255.231.251]) by orsmga006.jf.intel.com with ESMTP; 30 Apr 2020 15:20:07 -0700 Subject: Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible pages From: Dave Hansen To: Christian Borntraeger , Claudio Imbrenda , viro@zeniv.linux.org.uk Cc: david@redhat.com, akpm@linux-foundation.org, aarcange@redhat.com, linux-mm@kvack.org, frankja@linux.ibm.com, sfr@canb.auug.org.au, jhubbard@nvidia.com, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, jack@suse.cz, kirill@shutemov.name, peterz@infradead.org, sean.j.christopherson@intel.com, Ulrich.Weigand@de.ibm.com References: <20200430143825.3534128-1-imbrenda@linux.ibm.com> <1a3f5107-9847-73d4-5059-c6ef9d293551@de.ibm.com> Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org One other thing... The gup code will not take references on ref-frozen pages: > static inline __must_check bool try_get_page(struct page *page) > { > page = compound_head(page); > if (WARN_ON_ONCE(page_ref_count(page) <= 0)) > return false; > page_ref_inc(page); > return true; > } *But*, notice that the path that skips taking a ref is also a WARN_ON_ONCE(). Basically, if you get to try_get_page() on a ref-frozen page, it's considered buggy. This makes sense because you fundamentally can't freeze refs on a page that might have more refs taken on it. I think all the other users do this by ensuring that any PTE that could be gup'd is set non-present before the refs are frozen and remote TLBs are flushed which also ensures no GUPs are running. I don't know if the s390 code has some other way of preventing GUPs, but leaving Present=1 PTEs while you freeze refs would be quite troublesome on x86.