From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 3 Oct 2018 18:44:07 +0200 From: Jan Kara To: Dan Williams Cc: Jan Kara , Christoph Hellwig , Johannes Thumshirn , Dave Jiang , linux-nvdimm , Linux MM , linux-fsdevel , linux-ext4 , linux-xfs , Linux API Subject: Re: Problems with VM_MIXEDMAP removal from /proc//smaps Message-ID: <20181003164407.GK24030@quack2.suse.cz> References: <20181002142959.GD9127@quack2.suse.cz> <20181002143713.GA19845@infradead.org> <20181002144412.GC4963@linux-x5ow.site> <20181002145206.GA10903@infradead.org> <20181002153100.GG9127@quack2.suse.cz> <20181003125056.GA21043@quack2.suse.cz> <20181003150658.GC24030@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: On Wed 03-10-18 08:13:37, Dan Williams wrote: > On Wed, Oct 3, 2018 at 8:07 AM Jan Kara wrote: > > WRT per-inode DAX property, AFAIU that inode flag is just going to be > > advisory thing - i.e., use DAX if possible. If you mount a filesystem with > > these inode flags set in a configuration which does not allow DAX to be > > used, you will still be able to access such inodes but the access will use > > page cache instead. And querying these flags should better show real > > on-disk status and not just whether DAX is used as that would result in an > > even bigger mess. So this feature seems to be somewhat orthogonal to the > > API I'm looking for. > > True, I imagine once we have that flag we will be able to distinguish > the "saved" property and the "effective / live" property of DAX... > Also it's really not DAX that applications care about as much as "is > there page-cache indirection / overhead for this mapping?". That seems > to be a narrower guarantee that we can make than what "DAX" might > imply. Right. So what do people think about my suggestion earlier in the thread to use madvise(MADV_DIRECT_ACCESS) for this? Currently it would return success when DAX is in use, failure otherwise. Later we could extend it to be also used as a hint for caching policy for the inode... Honza -- Jan Kara SUSE Labs, CR