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29 Apr 2020 16:52:47 -0700 IronPort-SDR: 5PKddNU5qfNlbLPmLWP+BQ+Nwj2De5T4GlyeO6H6YH7Gv55n2RXNcpXSd0MnUCQfOzB0T3MyBJ 4F7wea+TFveg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,333,1583222400"; d="scan'208";a="261591798" Received: from isdasana-mobl1.amr.corp.intel.com (HELO [10.254.74.214]) ([10.254.74.214]) by orsmga006.jf.intel.com with ESMTP; 29 Apr 2020 16:52:46 -0700 Subject: Re: [PATCH v1 1/1] fs/splice: add missing callback for inaccessible pages To: Claudio Imbrenda Cc: Christian Borntraeger , akpm@linux-foundation.org, jack@suse.cz, kirill@shutemov.name, 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, Ulrich.Weigand@de.ibm.com References: <20200428225043.3091359-1-imbrenda@linux.ibm.com> <2a1abf38-d321-e3c7-c3b1-53b6db6da310@intel.com> <609afef2-43c2-d048-1c01-448a53a54d4e@intel.com> <20200430005310.7b25efab@p-imbrenda> From: Dave Hansen 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: <20200430005310.7b25efab@p-imbrenda> Content-Type: text/plain; charset=utf-8 Content-Language: en-US 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 4/29/20 3:53 PM, Claudio Imbrenda wrote: >> Actually, that's the problem. You've gone through all these careful >> checks and made the page inaccessible. *After* that process, how do >> you keep the page from being hit by an I/O device before it's made >> accessible again? My patch just assumes that *all* pages have gone >> through that process and passed those checks. > I don't understand what you are saying here. >=20 > we start with all pages accessible, we mark pages as inaccessible when > they are imported in the secure guest (we use the PG_arch_1 bit in > struct page). we then try to catch all I/O on inaccessible pages and > make them accessible so that I/O devices are happy.=20 The catching mechanism is incomplete, that's all I'm saying. Without looking too hard, and not even having the hardware, I've found two paths where the "catching" was incomplete: 1. sendfile(), which you've patched 2. sendto(), which you haven't patched > either your quick and dirty patch was too dirty (e.g. not accounting > for possible races between make_accessible/make_inaccessible), or some > of the functions in the trace you provided should do pin_user_page > instead of get_user_page (or both) I looked in the traces for any races. For sendto(), at least, the make_accessible() never happened before the process exited. That's entirely consistent with the theory that it entirely missed being caught. I can't find any evidence that there were races. Go ahead and try it. You have the patch! I mean, I found a bug in about 10 minutes in one tiny little VM. And, yes, you need to get rid of the FOLL_PIN check unless you want to go change a big swath of the remaining get_user_pages*() sites.