From: Dave Chinner <david@fromorbit.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Darrick J. Wong" <djwong@kernel.org>,
"ruansy.fnst@fujitsu.com" <ruansy.fnst@fujitsu.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"darrick.wong@oracle.com" <darrick.wong@oracle.com>,
"willy@infradead.org" <willy@infradead.org>,
"jack@suse.cz" <jack@suse.cz>,
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"hch@lst.de" <hch@lst.de>, "rgoldwyn@suse.de" <rgoldwyn@suse.de>,
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Subject: Re: Question about the "EXPERIMENTAL" tag for dax in XFS
Date: Sat, 27 Feb 2021 08:27:48 +1100 [thread overview]
Message-ID: <20210226212748.GY4662@dread.disaster.area> (raw)
In-Reply-To: <CAPcyv4iDefA3Y0wUW=p080SYAsM_2TPJba-V-sxdK_BeJMkmsw@mail.gmail.com>
On Fri, Feb 26, 2021 at 12:59:53PM -0800, Dan Williams wrote:
> On Fri, Feb 26, 2021 at 12:51 PM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Fri, Feb 26, 2021 at 11:24:53AM -0800, Dan Williams wrote:
> > > On Fri, Feb 26, 2021 at 11:05 AM Darrick J. Wong <djwong@kernel.org> wrote:
> > > >
> > > > On Fri, Feb 26, 2021 at 09:45:45AM +0000, ruansy.fnst@fujitsu.com wrote:
> > > > > Hi, guys
> > > > >
> > > > > Beside this patchset, I'd like to confirm something about the
> > > > > "EXPERIMENTAL" tag for dax in XFS.
> > > > >
> > > > > In XFS, the "EXPERIMENTAL" tag, which is reported in waring message
> > > > > when we mount a pmem device with dax option, has been existed for a
> > > > > while. It's a bit annoying when using fsdax feature. So, my initial
> > > > > intention was to remove this tag. And I started to find out and solve
> > > > > the problems which prevent it from being removed.
> > > > >
> > > > > As is talked before, there are 3 main problems. The first one is "dax
> > > > > semantics", which has been resolved. The rest two are "RMAP for
> > > > > fsdax" and "support dax reflink for filesystem", which I have been
> > > > > working on.
> > > >
> > > > <nod>
> > > >
> > > > > So, what I want to confirm is: does it means that we can remove the
> > > > > "EXPERIMENTAL" tag when the rest two problem are solved?
> > > >
> > > > Yes. I'd keep the experimental tag for a cycle or two to make sure that
> > > > nothing new pops up, but otherwise the two patchsets you've sent close
> > > > those two big remaining gaps. Thank you for working on this!
> > > >
> > > > > Or maybe there are other important problems need to be fixed before
> > > > > removing it? If there are, could you please show me that?
> > > >
> > > > That remains to be seen through QA/validation, but I think that's it.
> > > >
> > > > Granted, I still have to read through the two patchsets...
> > >
> > > I've been meaning to circle back here as well.
> > >
> > > My immediate concern is the issue Jason recently highlighted [1] with
> > > respect to invalidating all dax mappings when / if the device is
> > > ripped out from underneath the fs. I don't think that will collide
> > > with Ruan's implementation, but it does need new communication from
> > > driver to fs about removal events.
> > >
> > > [1]: http://lore.kernel.org/r/CAPcyv4i+PZhYZiePf2PaH0dT5jDfkmkDX-3usQy1fAhf6LPyfw@mail.gmail.com
> >
> > Oh, yay.
> >
> > The XFS shutdown code is centred around preventing new IO from being
> > issued - we don't actually do anything about DAX mappings because,
> > well, I don't think anyone on the filesystem side thought they had
> > to do anything special if pmem went away from under it.
> >
> > My understanding -was- that the pmem removal invalidates
> > all the ptes currently mapped into CPU page tables that point at
> > the dax device across the system. THe vmas that manage these
> > mappings are not really something the filesystem really manages,
> > but a function of the mm subsystem. What the filesystem cares about
> > is that it gets page faults triggered when a change of state occurs
> > so that it can remap the page to it's backing store correctly.
> >
> > IOWs, all the mm subsystem needs to when pmem goes away is clear the
> > CPU ptes, because then when then when userspace tries to access the
> > mapped DAX pages we get a new page fault. In processing the fault, the
> > filesystem will try to get direct access to the pmem from the block
> > device. This will get an ENODEV error from the block device because
> > because the backing store (pmem) has been unplugged and is no longer
> > there...
> >
> > AFAICT, as long as pmem removal invalidates all the active ptes that
> > point at the pmem being removed, the filesystem doesn't need to
> > care about device removal at all, DAX or no DAX...
>
> How would the pmem removal do that without walking all the active
> inodes in the fs at the time of shutdown and call
> unmap_mapping_range(inode->i_mapping, 0, 0, 1)?
Which then immediately ends up back at the vmas that manage the ptes
to unmap them.
Isn't finding the vma(s) that map a specific memory range exactly
what the rmap code in the mm subsystem is supposed to address?
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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next prev parent reply other threads:[~2021-02-26 21:27 UTC|newest]
Thread overview: 71+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-26 0:20 [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax Shiyang Ruan
2021-02-26 0:20 ` [PATCH v2 01/10] fsdax: Factor helpers to simplify dax fault code Shiyang Ruan
2021-03-03 9:13 ` Christoph Hellwig
2021-02-26 0:20 ` [PATCH v2 02/10] fsdax: Factor helper: dax_fault_actor() Shiyang Ruan
2021-03-03 9:28 ` Christoph Hellwig
2021-03-12 9:01 ` ruansy.fnst
2021-02-26 0:20 ` [PATCH v2 03/10] fsdax: Output address in dax_iomap_pfn() and rename it Shiyang Ruan
2021-02-26 0:20 ` [PATCH v2 04/10] fsdax: Introduce dax_iomap_cow_copy() Shiyang Ruan
2021-03-03 9:29 ` Christoph Hellwig
2021-02-26 0:20 ` [PATCH v2 05/10] fsdax: Replace mmap entry in case of CoW Shiyang Ruan
2021-03-03 9:30 ` Christoph Hellwig
2021-03-03 9:41 ` ruansy.fnst
2021-03-03 9:44 ` Christoph Hellwig
2021-03-03 9:48 ` Christoph Hellwig
2021-02-26 0:20 ` [PATCH v2 06/10] fsdax: Add dax_iomap_cow_copy() for dax_iomap_zero Shiyang Ruan
2021-03-03 9:31 ` Christoph Hellwig
2021-02-26 0:20 ` [PATCH v2 07/10] iomap: Introduce iomap_apply2() for operations on two files Shiyang Ruan
2021-02-26 4:14 ` Darrick J. Wong
2021-02-26 8:11 ` ruansy.fnst
2021-02-26 8:25 ` Shiyang Ruan
2021-03-04 5:41 ` [RESEND PATCH v2.1 " Shiyang Ruan
2021-03-11 12:30 ` Christoph Hellwig
2021-02-26 0:20 ` [PATCH v2 08/10] fsdax: Dedup file range to use a compare function Shiyang Ruan
2021-02-26 8:28 ` Shiyang Ruan
2021-03-03 8:20 ` Joe Perches
2021-03-04 5:42 ` [RESEND PATCH v2.1 " Shiyang Ruan
2021-02-26 0:20 ` [PATCH v2 09/10] fs/xfs: Handle CoW for fsdax write() path Shiyang Ruan
2021-03-03 9:43 ` Christoph Hellwig
2021-03-03 9:57 ` ruansy.fnst
2021-03-03 10:43 ` Christoph Hellwig
2021-03-04 1:35 ` ruansy.fnst
2021-02-26 0:20 ` [PATCH v2 10/10] fs/xfs: Add dedupe support for fsdax Shiyang Ruan
2021-02-26 9:45 ` Question about the "EXPERIMENTAL" tag for dax in XFS ruansy.fnst
2021-02-26 19:04 ` Darrick J. Wong
2021-02-26 19:24 ` Dan Williams
2021-02-26 20:51 ` Dave Chinner
2021-02-26 20:59 ` Dan Williams
2021-02-26 21:27 ` Dave Chinner [this message]
2021-02-26 22:41 ` Dan Williams
2021-02-27 22:36 ` Dave Chinner
2021-02-27 23:40 ` Dan Williams
2021-02-28 22:38 ` Dave Chinner
2021-03-01 20:55 ` Dan Williams
2021-03-01 22:46 ` Dave Chinner
2021-03-02 0:32 ` Dan Williams
2021-03-02 2:42 ` Dave Chinner
2021-03-02 3:33 ` Dan Williams
2021-03-02 5:38 ` Dave Chinner
2021-03-02 5:50 ` Dan Williams
2021-03-02 3:28 ` Darrick J. Wong
2021-03-02 5:41 ` Dan Williams
2021-03-02 7:57 ` Dave Chinner
2021-03-02 17:49 ` Dan Williams
2021-03-04 23:40 ` Darrick J. Wong
2021-03-01 7:26 ` Yasunori Goto
2021-03-01 21:34 ` Dan Williams
2021-03-09 6:36 ` [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax Xiaoguang Wang
2021-03-09 16:19 ` Goldwyn Rodrigues
2021-03-10 1:26 ` ruansy.fnst
2021-03-10 12:30 ` Neal Gompa
2021-03-10 13:02 ` Matthew Wilcox
2021-03-10 13:36 ` Neal Gompa
2021-03-10 13:55 ` Matthew Wilcox
2021-03-10 14:21 ` Goldwyn Rodrigues
2021-03-10 14:26 ` Matthew Wilcox
2021-03-10 17:04 ` Goldwyn Rodrigues
2021-03-11 0:53 ` Dan Williams
2021-03-11 8:26 ` Neal Gompa
2021-03-13 13:07 ` Adam Borowski
2021-03-13 16:24 ` Neal Gompa
2021-03-13 22:00 ` Adam Borowski
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