From: Dave Chinner <david@fromorbit.com>
To: Jan Kara <jack@suse.cz>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
Jeff Moyer <jmoyer@redhat.com>, Mike Snitzer <snitzer@redhat.com>,
"Kani, Toshi" <toshi.kani@hpe.com>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
"dm-devel@redhat.com" <dm-devel@redhat.com>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"ross.zwisler@linux.intel.com" <ross.zwisler@linux.intel.com>,
"dan.j.williams@intel.com" <dan.j.williams@intel.com>
Subject: Re: Snapshot target and DAX-capable devices
Date: Wed, 5 Sep 2018 11:25:57 +1000 [thread overview]
Message-ID: <20180905012557.GE27618@dastard> (raw)
In-Reply-To: <20180831094255.GB11622@quack2.suse.cz>
On Fri, Aug 31, 2018 at 11:42:55AM +0200, Jan Kara wrote:
> On Fri 31-08-18 09:38:09, Dave Chinner wrote:
> > On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote:
> > > You can't support dax on snapshot - if someone maps a block and the block
> > > needs to be moved, then what?
> >
> > This is only a problem for access via mmap and page faults.
....
> > It's a whole different ballgame for a dm-snapshot device - block
> > devices are completely unaware of page faults to DAX file mappings.
>
> Actually, block devices are not completely unaware of DAX page faults -
> they will get ->direct_access callback for the fault range. It does not
> currently convey enough information - we also need to inform the block
> device whether it is read or write. But that's about all that's needed to
> add AFAICT. And by comparing returned PFN with the one we have stored in
> the radix tree (which we have if that file offset is mapped by anybody),
> the filesystem / DAX code can tell whether remapping happened and do the
> unmapping.
I forgot about the direct access call.
But it seems like a hack to redefine the simple, fast sector-to-pfn
translation into a slow and potentially resource hungry interface
for physical storage reallocation. Doing storage layer COW
operations inside direct_access takes us straight back to the bad
ways of get_block() interfaces. We moved all the filesystem
allocation to iomap so that the storage management is separated from
the mm/physical address translation side of DAX - doing block device
storage management operations inside ->direct_access effectively
reverts that separation and so just seems like a hack to me.
Oh, right, DAX. Silly me. :/
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2018-09-05 5:53 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-27 16:07 Snapshot target and DAX-capable devices Jan Kara
2018-08-27 16:43 ` Kani, Toshi
2018-08-28 7:50 ` Jan Kara
2018-08-28 17:56 ` Mike Snitzer
2018-08-28 22:38 ` Kani, Toshi
2018-08-30 9:30 ` Jan Kara
2018-08-30 18:49 ` Mike Snitzer
2018-08-30 19:32 ` Jeff Moyer
2018-08-30 19:47 ` Mikulas Patocka
2018-08-30 19:53 ` Jeff Moyer
2018-08-30 23:38 ` Dave Chinner
2018-08-31 9:42 ` Jan Kara
2018-09-05 1:25 ` Dave Chinner [this message]
2018-12-12 16:11 ` Huaisheng Ye
2018-12-12 16:12 ` Christoph Hellwig
2018-12-12 17:50 ` Mike Snitzer
2018-12-12 19:49 ` Kani, Toshi
2018-12-12 21:15 ` Theodore Y. Ts'o
2018-12-12 22:43 ` Mike Snitzer
2018-12-14 4:11 ` [dm-devel] " Theodore Y. Ts'o
2018-12-14 8:24 ` [External] " Huaisheng HS1 Ye
2018-12-18 19:49 ` Mike Snitzer
2018-08-30 19:44 ` Mikulas Patocka
2018-08-31 10:01 ` Jan Kara
2018-08-30 22:55 ` Dave Chinner
2018-08-31 9:54 ` Jan Kara
2018-08-30 19:17 ` [dm-devel] " Jeff Moyer
2018-08-31 9:14 ` Jan Kara
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180905012557.GE27618@dastard \
--to=david@fromorbit.com \
--cc=dan.j.williams@intel.com \
--cc=dm-devel@redhat.com \
--cc=jack@suse.cz \
--cc=jmoyer@redhat.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-nvdimm@lists.01.org \
--cc=mpatocka@redhat.com \
--cc=ross.zwisler@linux.intel.com \
--cc=snitzer@redhat.com \
--cc=toshi.kani@hpe.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).