Return-Path: Received: from mail-oi1-f193.google.com ([209.85.167.193]:41294 "EHLO mail-oi1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727148AbeJCDEL (ORCPT ); Tue, 2 Oct 2018 23:04:11 -0400 Received: by mail-oi1-f193.google.com with SMTP id l197-v6so2628749oib.8 for ; Tue, 02 Oct 2018 13:19:05 -0700 (PDT) MIME-Version: 1.0 References: <20181002100531.GC4135@quack2.suse.cz> <20181002121039.GA3274@linux-x5ow.site> <20181002142959.GD9127@quack2.suse.cz> <20181002143713.GA19845@infradead.org> <20181002144412.GC4963@linux-x5ow.site> <20181002145206.GA10903@infradead.org> <20181002153100.GG9127@quack2.suse.cz> In-Reply-To: <20181002153100.GG9127@quack2.suse.cz> From: Dan Williams Date: Tue, 2 Oct 2018 13:18:54 -0700 Message-ID: Subject: Re: Problems with VM_MIXEDMAP removal from /proc//smaps To: Jan Kara Cc: Christoph Hellwig , Johannes Thumshirn , Dave Jiang , linux-nvdimm , Linux MM , linux-fsdevel , linux-ext4 , linux-xfs , Linux API Content-Type: text/plain; charset="UTF-8" Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, Oct 2, 2018 at 8:32 AM Jan Kara wrote: > > On Tue 02-10-18 07:52:06, Christoph Hellwig wrote: > > On Tue, Oct 02, 2018 at 04:44:13PM +0200, Johannes Thumshirn wrote: > > > On Tue, Oct 02, 2018 at 07:37:13AM -0700, Christoph Hellwig wrote: > > > > No, it should not. DAX is an implementation detail thay may change > > > > or go away at any time. > > > > > > Well we had an issue with an application checking for dax, this is how > > > we landed here in the first place. > > > > So what exacty is that "DAX" they are querying about (and no, I'm not > > joking, nor being philosophical). > > I believe the application we are speaking about is mostly concerned about > the memory overhead of the page cache. Think of a machine that has ~ 1TB of > DRAM, the database running on it is about that size as well and they want > database state stored somewhere persistently - which they may want to do by > modifying mmaped database files if they do small updates... So they really > want to be able to use close to all DRAM for the DB and not leave slack > space for the kernel page cache to cache 1TB of database files. VM_MIXEDMAP was never a reliable indication of DAX because it could be set for random other device-drivers that use vm_insert_mixed(). The MAP_SYNC flag positively indicates that page cache is disabled for a given mapping, although whether that property is due to "dax" or some other kernel mechanics is purely an internal detail. I'm not opposed to faking out VM_MIXEDMAP if this broken check has made it into production, but again, it's unreliable.