Re: Question about the "EXPERIMENTAL" tag for dax in XFS

[Dan Williams]

On Fri, Feb 26, 2021 at 1:28 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Fri, Feb 26, 2021 at 09:45:45AM +0000, ruansy.fnst@xxxxxxxxxxx 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@xxxxxxxxxxxxxx
> > >
> > > 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?

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.