From: Nicholas Piggin <npiggin@gmail.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Oliver O'Halloran <oohall@gmail.com>,
Yumei Huang <yuhuang@redhat.com>, Michal Hocko <mhocko@suse.com>,
Xiao Guangrong <guangrong.xiao@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
Gleb Natapov <gleb@kernel.org>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
mtosatti@redhat.com,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Dave Hansen <dave.hansen@intel.com>,
Stefan Hajnoczi <stefanha@redhat.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps)
Date: Tue, 13 Sep 2016 11:53:11 +1000 [thread overview]
Message-ID: <20160913115311.509101b0@roar.ozlabs.ibm.com> (raw)
In-Reply-To: <20160912213435.GD30497@dastard>
On Tue, 13 Sep 2016 07:34:36 +1000
Dave Chinner <david@fromorbit.com> wrote:
> On Mon, Sep 12, 2016 at 06:05:07PM +1000, Nicholas Piggin wrote:
> > On Mon, 12 Sep 2016 00:51:28 -0700
> > Christoph Hellwig <hch@infradead.org> wrote:
> >
> > > On Mon, Sep 12, 2016 at 05:25:15PM +1000, Oliver O'Halloran wrote:
> > > > What are the problems here? Is this a matter of existing filesystems
> > > > being unable/unwilling to support this or is it just fundamentally
> > > > broken?
> > >
> > > It's a fundamentally broken model. See Dave's post that actually was
> > > sent slightly earlier then mine for the list of required items, which
> > > is fairly unrealistic. You could probably try to architect a file
> > > system for it, but I doubt it would gain much traction.
> >
> > It's not fundamentally broken, it just doesn't fit well existing
> > filesystems.
> >
> > Dave's post of requirements is also wrong. A filesystem does not have
> > to guarantee all that, it only has to guarantee that is the case for
> > a given block after it has a mapping and page fault returns, other
> > operations can be supported by invalidating mappings, etc.
>
> Sure, but filesystems are completely unaware of what is mapped at
> any given time, or what constraints that mapping might have. Trying
> to make filesystems aware of per-page mapping constraints seems like
I'm not sure what you mean. The filesystem can hand out mappings
and fault them in itself. It can invalidate them.
> a fairly significant layering violation based on a flawed
> assumption. i.e. that operations on other parts of the file do not
> affect the block that requires immutable metadata.
>
> e.g an extent operation in some other area of the file can cause a
> tip-to-root extent tree split or merge, and that moves the metadata
> that points to the mapped block that we've told userspace "doesn't
> need fsync". We now need an fsync to ensure that the metadata is
> consistent on disk again, even though that block has not physically
> been moved.
You don't, because the filesystem can invalidate existing mappings
and do the right thing when they are faulted in again. That's the
big^Wmedium hammer approach that can cope with most problems.
But let me understand your example in the absence of that.
- Application mmaps a file, faults in block 0
- FS allocates block, creates mappings, syncs metadata, sets "no fsync"
flag for that block, and completes the fault.
- Application writes some data to block 0, completes userspace flushes
* At this point, a crash must return with above data (or newer).
- Application starts writing more stuff into block 0
- Concurrently, fault in block 1
- FS starts to allocate, splits trees including mappings to block 0
* Crash
Is that right? How does your filesystem lose data before the sync
point?
> IOWs, the immutable data block updates are now not
> ordered correctly w.r.t. other updates done to the file, especially
> when we consider crash recovery....
>
> All this will expose is an unfixable problem with ordering of stable
> data + metadata operations and their synchronisation. As such, it
> seems like nothing but a major cluster-fuck to try to do mapping
> specific, per-block immutable metadata - it adds major complexity
> and even more untractable problems.
>
> Yes, we /could/ try to solve this but, quite frankly, it's far
> easier to change the broken PMEM programming model assumptions than
> it is to implement what you are suggesting. Or to do what Christoph
> suggested and just use a wrapper around something like device
> mapper to hand out chunks of unchanging, static pmem to
> applications...
If there is any huge complexity or unsolved problem, it is in XFS.
Conceptual problem is simple.
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next prev parent reply other threads:[~2016-09-13 1:53 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-08 4:32 DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps) Dan Williams
2016-09-08 22:56 ` Ross Zwisler
2016-09-08 23:04 ` Dan Williams
2016-09-09 8:55 ` Xiao Guangrong
2016-09-09 15:40 ` Dan Williams
2016-09-12 6:00 ` Xiao Guangrong
2016-09-12 3:44 ` Rudoff, Andy
2016-09-12 6:31 ` Xiao Guangrong
2016-09-12 1:40 ` Dave Chinner
2016-09-15 5:55 ` Darrick J. Wong
2016-09-15 6:25 ` Dave Chinner
2016-09-12 5:27 ` Christoph Hellwig
2016-09-12 7:25 ` Oliver O'Halloran
2016-09-12 7:51 ` Christoph Hellwig
2016-09-12 8:05 ` Nicholas Piggin
2016-09-12 15:01 ` Christoph Hellwig
2016-09-13 1:31 ` Nicholas Piggin
2016-09-13 4:06 ` Dan Williams
2016-09-13 5:40 ` Nicholas Piggin
2016-09-12 21:34 ` Dave Chinner
2016-09-13 1:53 ` Nicholas Piggin [this message]
2016-09-13 7:17 ` Christoph Hellwig
2016-09-13 9:06 ` Nicholas Piggin
2016-09-14 7:39 ` Dave Chinner
2016-09-14 10:19 ` Nicholas Piggin
2016-09-15 2:31 ` Dave Chinner
2016-09-15 3:49 ` Nicholas Piggin
2016-09-15 10:32 ` Dave Chinner
2016-09-15 11:42 ` Nicholas Piggin
2016-09-15 22:33 ` Dave Chinner
2016-09-16 5:54 ` Nicholas Piggin
2016-12-19 21:11 ` Ross Zwisler
2016-12-20 1:09 ` Darrick J. Wong
2016-12-20 1:18 ` Dan Williams
2016-12-21 0:40 ` Darrick J. Wong
2016-12-21 16:53 ` Dan Williams
2016-12-21 21:24 ` Dave Chinner
2016-12-21 21:33 ` Dan Williams
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