From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 2 Oct 2018 17:01:24 +0200 From: Johannes Thumshirn To: Christoph Hellwig Cc: Jan Kara , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org, mhocko@suse.cz, Dan Williams Subject: Re: Problems with VM_MIXEDMAP removal from /proc//smaps Message-ID: <20181002150123.GD4963@linux-x5ow.site> References: <20181002100531.GC4135@quack2.suse.cz> <20181002121039.GA3274@linux-x5ow.site> <20181002142010.GB4963@linux-x5ow.site> <20181002144547.GA26735@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20181002144547.GA26735@infradead.org> Sender: owner-linux-mm@kvack.org List-ID: On Tue, Oct 02, 2018 at 07:45:47AM -0700, Christoph Hellwig wrote: > How does an application "make use of DAX"? What actual user visible > semantics are associated with a file that has this flag set? There may not be any user visible semantics of DAX, but there are promises we gave to application developers praising DAX as _the_ method to map data on persistent memory and get around "the penalty of the page cache" (however big this is). As I said in another mail to this thread, applications have started to poke in procfs to see whether they can use DAX or not. Party A has promised party B something and they started checking for it, then commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and device dax" removed the way they checked if the kernel can keep up with this promise. So technically e1fb4a086495 is a user visible regression and in the past we have reverted patches introducing these, even if the patch is generally correct and poking in /proc/self/smaps is a bad idea. I just wanted to give them a documented way to check for this promise. Being neutral if this promise is right or wrong, good or bad, or whatever. That's not my call, but I prefer not having angry users, yelling at me because of broken applications. Byte, Johannes -- Johannes Thumshirn Storage jthumshirn@suse.de +49 911 74053 689 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 N�rnberg GF: Felix Imend�rffer, Jane Smithard, Graham Norton HRB 21284 (AG N�rnberg) Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850