From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id 8858A6B03BD for ; Wed, 21 Dec 2016 11:53:48 -0500 (EST) Received: by mail-oi0-f72.google.com with SMTP id v84so395205088oie.0 for ; Wed, 21 Dec 2016 08:53:48 -0800 (PST) Received: from mail-oi0-x235.google.com (mail-oi0-x235.google.com. [2607:f8b0:4003:c06::235]) by mx.google.com with ESMTPS id c191si13552839oih.34.2016.12.21.08.53.47 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 21 Dec 2016 08:53:47 -0800 (PST) Received: by mail-oi0-x235.google.com with SMTP id b126so216021179oia.2 for ; Wed, 21 Dec 2016 08:53:47 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20161221004031.GF9865@birch.djwong.org> References: <20160914201936.08315277@roar.ozlabs.ibm.com> <20160915023133.GR22388@dastard> <20160915134945.0aaa4f5a@roar.ozlabs.ibm.com> <20160915103210.GT22388@dastard> <20160915214222.505f4888@roar.ozlabs.ibm.com> <20160915223350.GU22388@dastard> <20160916155405.6b634bbc@roar.ozlabs.ibm.com> <20161219211149.GA12822@linux.intel.com> <20161220010936.GH7311@birch.djwong.org> <20161221004031.GF9865@birch.djwong.org> From: Dan Williams Date: Wed, 21 Dec 2016 08:53:46 -0800 Message-ID: Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps) Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: "Darrick J. Wong" Cc: Ross Zwisler , Nicholas Piggin , Dave Chinner , Jan Kara , Jeff Moyer , Yumei Huang , Michal Hocko , Xiaof Guangrong , KVM list , Dave Hansen , Gleb Natapov , "linux-nvdimm@lists.01.org" , mtosatti@redhat.com, "linux-kernel@vger.kernel.org" , Christoph Hellwig , Linux MM , Stefan Hajnoczi , linux-fsdevel , Paolo Bonzini , Andrew Morton On Tue, Dec 20, 2016 at 4:40 PM, Darrick J. Wong wrote: > On Mon, Dec 19, 2016 at 05:18:40PM -0800, Dan Williams wrote: >> On Mon, Dec 19, 2016 at 5:09 PM, Darrick J. Wong >> wrote: >> > On Mon, Dec 19, 2016 at 02:11:49PM -0700, Ross Zwisler wrote: >> >> On Fri, Sep 16, 2016 at 03:54:05PM +1000, Nicholas Piggin wrote: >> >> <> >> >> > Definitely the first step would be your simple preallocated per >> >> > inode approach until it is shown to be insufficient. >> >> >> >> Reviving this thread a few months later... >> >> >> >> Dave, we're interested in taking a serious look at what it would take to get >> >> PMEM_IMMUTABLE working. Do you still hold the opinion that this is (or could >> >> become, with some amount of work) a workable solution? >> >> >> >> We're happy to do the grunt work for this feature, but we will probably need >> >> guidance from someone with more XFS experience. With you out on extended leave >> >> the first half of 2017, who would be the best person to ask for this guidance? >> >> Darrick? >> > >> > Yes, probably. :) >> > >> > I think where we left off with this (on the XFS side) is some sort of >> > fallocate mode that would allocate blocks, zero them, and then set the >> > DAX and PMEM_IMMUTABLE on-disk inode flags. After that, you'd mmap the >> > file and thereby gain the ability to control write persistents behavior >> > without having to worry about fs metadata updates. As an added plus, I >> > think zeroing the pmem also clears media errors, or something like that. >> > >> > Is that a reasonable starting point? My memory is a little foggy. >> > >> > Hmm, I see Dan just posted something about blockdev fallocate. I'll go >> > read that. >> >> That's for device-dax, which is basically a poor man's PMEM_IMMUTABLE >> via a character device interface. It's useful for cases where you want >> an entire nvdimm namespace/volume in "no fs-metadata to worry about" >> mode. But, for sub-allocations of a namespace and support for >> existing tooling, PMEM_IMMUTABLE is much more usable. > > Well sure... but otoh I was thinking that it'd be pretty neat if we > could use the same code regardless of whether the target file was a > dax-device or an xfs file: > > fd = open("", O_RDWR); > fstat(fd, &statbuf): > fallocate(fd, FALLOC_FL_PMEM_IMMUTABLE, 0, statbuf.st_size); > p = mmap(NULL, statbuf.st_size, PROT_READ | PROT_WRITE, fd, 0); > > *(p + 42) = 0xDEADBEEF; > asm { clflush; } /* or whatever */ > > ...so perhaps it would be a good idea to design the fallocate primitive > around "prepare this fd for mmap-only pmem semantics" and let it the > backend do zeroing and inode flag changes as necessary to make it > happen. We'd need to do some bikeshedding about what the other falloc > flags mean when we're dealing with pmem files and devices, but I think > we should try to keep the userland presentation the same unless there's > a really good reason not to. It would be interesting to use fallocate to size device-dax files... However, the fallocate userland presentation I was looking to unify was the semantic for how to clear errors. The current method for creating a device-dax instance is 'echo "" > ""'. Christoph put the kibosh on carrying the fallocate-error-clearing semantic over, so we'll need to fallback to using the existing ND_IOCTL_CLEAR_ERROR. All that's missing for error clearing is a secondary mechanism to report the media error list for a physical address range. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org