Re: Question about the "EXPERIMENTAL" tag for dax in XFS
From: Dan Williams
Date: Fri Feb 26 2021 - 17:43:11 EST
On Fri, Feb 26, 2021 at 1:28 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
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On Fri, Feb 26, 2021 at 12:59:53PM -0800, Dan Williams wrote:
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> On Fri, Feb 26, 2021 at 12:51 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
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> >
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> > On Fri, Feb 26, 2021 at 11:24:53AM -0800, Dan Williams wrote:
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> > > On Fri, Feb 26, 2021 at 11:05 AM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
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> > > >
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> > > > On Fri, Feb 26, 2021 at 09:45:45AM +0000, ruansy.fnst@xxxxxxxxxxx wrote:
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> > > > > Hi, guys
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> > > > >
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> > > > > Beside this patchset, I'd like to confirm something about the
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> > > > > "EXPERIMENTAL" tag for dax in XFS.
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> > > > >
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> > > > > In XFS, the "EXPERIMENTAL" tag, which is reported in waring message
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> > > > > when we mount a pmem device with dax option, has been existed for a
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> > > > > while. It's a bit annoying when using fsdax feature. So, my initial
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> > > > > intention was to remove this tag. And I started to find out and solve
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> > > > > the problems which prevent it from being removed.
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> > > > >
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> > > > > As is talked before, there are 3 main problems. The first one is "dax
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> > > > > semantics", which has been resolved. The rest two are "RMAP for
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> > > > > fsdax" and "support dax reflink for filesystem", which I have been
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> > > > > working on.
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> > > >
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> > > > <nod>
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> > > >
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> > > > > So, what I want to confirm is: does it means that we can remove the
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> > > > > "EXPERIMENTAL" tag when the rest two problem are solved?
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> > > >
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> > > > Yes. I'd keep the experimental tag for a cycle or two to make sure that
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> > > > nothing new pops up, but otherwise the two patchsets you've sent close
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> > > > those two big remaining gaps. Thank you for working on this!
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> > > >
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> > > > > Or maybe there are other important problems need to be fixed before
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> > > > > removing it? If there are, could you please show me that?
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> > > >
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> > > > That remains to be seen through QA/validation, but I think that's it.
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> > > >
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> > > > Granted, I still have to read through the two patchsets...
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> > >
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> > > I've been meaning to circle back here as well.
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> > >
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> > > My immediate concern is the issue Jason recently highlighted [1] with
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> > > respect to invalidating all dax mappings when / if the device is
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> > > ripped out from underneath the fs. I don't think that will collide
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> > > with Ruan's implementation, but it does need new communication from
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> > > driver to fs about removal events.
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> > >
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> > > [1]: http://lore.kernel.org/r/CAPcyv4i+PZhYZiePf2PaH0dT5jDfkmkDX-3usQy1fAhf6LPyfw@xxxxxxxxxxxxxx
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> >
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> > Oh, yay.
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> >
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> > The XFS shutdown code is centred around preventing new IO from being
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> > issued - we don't actually do anything about DAX mappings because,
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> > well, I don't think anyone on the filesystem side thought they had
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> > to do anything special if pmem went away from under it.
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> >
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> > My understanding -was- that the pmem removal invalidates
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> > all the ptes currently mapped into CPU page tables that point at
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> > the dax device across the system. THe vmas that manage these
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> > mappings are not really something the filesystem really manages,
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> > but a function of the mm subsystem. What the filesystem cares about
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> > is that it gets page faults triggered when a change of state occurs
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> > so that it can remap the page to it's backing store correctly.
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> >
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> > IOWs, all the mm subsystem needs to when pmem goes away is clear the
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> > CPU ptes, because then when then when userspace tries to access the
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> > mapped DAX pages we get a new page fault. In processing the fault, the
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> > filesystem will try to get direct access to the pmem from the block
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> > device. This will get an ENODEV error from the block device because
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> > because the backing store (pmem) has been unplugged and is no longer
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> > there...
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> >
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> > AFAICT, as long as pmem removal invalidates all the active ptes that
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> > point at the pmem being removed, the filesystem doesn't need to
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> > care about device removal at all, DAX or no DAX...
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> How would the pmem removal do that without walking all the active
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> inodes in the fs at the time of shutdown and call
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> unmap_mapping_range(inode->i_mapping, 0, 0, 1)?
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Which then immediately ends up back at the vmas that manage the ptes
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to unmap them.
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Isn't finding the vma(s) that map a specific memory range exactly
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what the rmap code in the mm subsystem is supposed to address?
rmap can lookup only vmas from a virt address relative to a given
mm_struct. The driver has neither the list of mm_struct objects nor
virt addresses to do a lookup. All it knows is that someone might have
mapped pages through the fsdax interface.
To me this looks like a notifier that fires from memunmap_pages()
after dev_pagemap_kill() to notify any block_device associated with
that dev_pagemap() to say that any dax mappings arranged through this
block_device are now invalid. The reason to do this after
dev_pagemap_kill() is so that any new mapping attempts that are racing
the removal will be blocked.
The receiver of that notification needs to go from a block_device to a
superblock that has mapped inodes and walk ->sb_inodes triggering the
unmap/invalidation.