From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751523AbaEVCeP (ORCPT ); Wed, 21 May 2014 22:34:15 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:51751 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750796AbaEVCeO (ORCPT ); Wed, 21 May 2014 22:34:14 -0400 Date: Wed, 21 May 2014 19:33:36 -0700 From: Andrew Morton To: Naoya Horiguchi Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Konstantin Khlebnikov , Wu Fengguang , Arnaldo Carvalho de Melo , Borislav Petkov Subject: Re: [PATCH 0/4] pagecache scanning with /proc/kpagecache Message-Id: <20140521193336.5df90456.akpm@linux-foundation.org> In-Reply-To: <537d5ee4.4914e00a.5672.ffff85d5SMTPIN_ADDED_BROKEN@mx.google.com> References: <1400639194-3743-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20140521154250.95bc3520ad8d192d95efe39b@linux-foundation.org> <537d5ee4.4914e00a.5672.ffff85d5SMTPIN_ADDED_BROKEN@mx.google.com> X-Mailer: Sylpheed 2.7.1 (GTK+ 2.18.9; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 21 May 2014 22:19:55 -0400 Naoya Horiguchi wrote: > > A much nicer interface would be for us to (finally!) implement > > fincore(), perhaps with an enhanced per-present-page payload which > > presents the info which you need (although we don't actually know what > > that info is!). > > page/pfn of each page slot and its page cache tag as shown in patch 4/4. > > > This would require open() - it appears to be a requirement that the > > caller not open the file, but no reason was given for this. > > > > Requiring open() would address some of the obvious security concerns, > > but it will still be possible for processes to poke around and get some > > understanding of the behaviour of other processes. Careful attention > > should be paid to this aspect of any such patchset. > > Sorry if I missed your point, but this interface defines fixed mapping > between file position in /proc/kpagecache and in-file page offset of > the target file. So we do not need to use seq_file mechanism, that's > why open() is not defined and default one is used. > The same thing is true for /proc/{kpagecount,kpageflags}, from which > I copied/pasted some basic code. I think you did miss my point ;) Please do a web search for fincore - it's a syscall similar to mincore(), only it queries pagecache: fincore(int fd, loff_t offset, ...). In its simplest form it queries just for present/absent, but we could increase the query payload to incorporate additional per-page info. It would take a lot of thought and discussion to nail down the fincore() interface (we've already tried a couple of times). But unfortunately, fincore() is probably going to be implemented one day and it will (or at least could) make /proc/kpagecache obsolete.