From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:57390 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726524AbgD2Rbs (ORCPT ); Wed, 29 Apr 2020 13:31:48 -0400 Subject: Re: [PATCH v1 1/1] fs/splice: add missing callback for inaccessible pages References: <20200428225043.3091359-1-imbrenda@linux.ibm.com> <2a1abf38-d321-e3c7-c3b1-53b6db6da310@intel.com> From: Christian Borntraeger Message-ID: Date: Wed, 29 Apr 2020 19:31:22 +0200 MIME-Version: 1.0 In-Reply-To: <2a1abf38-d321-e3c7-c3b1-53b6db6da310@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-s390-owner@vger.kernel.org List-ID: To: Dave Hansen , Claudio Imbrenda , akpm@linux-foundation.org, jack@suse.cz, kirill@shutemov.name Cc: david@redhat.com, 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, peterz@infradead.org, sean.j.christopherson@intel.com On 29.04.20 18:07, Dave Hansen wrote: > On 4/28/20 3:50 PM, Claudio Imbrenda wrote: >> If a page is inaccesible and it is used for things like sendfile, then >> the content of the page is not always touched, and can be passed >> directly to a driver, causing issues. >> >> This patch fixes the issue by adding a call to arch_make_page_accessible >> in page_cache_pipe_buf_confirm; this fixes the issue. > > I spent about 5 minutes putting together a patch: > > https://sr71.net/~dave/intel/accessible.patch > > It adds a page flag ("daccess") which starts out set. It clears the > flag it when the page is added to the page cache or mapped as anonymous. And that of course does not work. Pages are not made unaccessible at a random point in time. We do check for several page flags and page count before doing so and we also do this while with paqe_ref_freeze to avoid several races. I guess you just hit one of those.