Re: åå: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink

[Darrick J. Wong]

On Tue, Apr 28, 2020 at 09:24:41PM +1000, Dave Chinner wrote:
> On Tue, Apr 28, 2020 at 04:16:36AM -0700, Matthew Wilcox wrote:
> > On Tue, Apr 28, 2020 at 05:32:41PM +0800, Ruan Shiyang wrote:
> > > On 2020/4/28 äå2:43, Dave Chinner wrote:
> > > > On Tue, Apr 28, 2020 at 06:09:47AM +0000, Ruan, Shiyang wrote:
> > > > > å 2020/4/27 20:28:36, "Matthew Wilcox" <willy@xxxxxxxxxxxxx> åé:
> > > > > > On Mon, Apr 27, 2020 at 04:47:42PM +0800, Shiyang Ruan wrote:
> > > > > > >   This patchset is a try to resolve the shared 'page cache' problem for
> > > > > > >   fsdax.
> > > > > > > 
> > > > > > >   In order to track multiple mappings and indexes on one page, I
> > > > > > >   introduced a dax-rmap rb-tree to manage the relationship.  A dax entry
> > > > > > >   will be associated more than once if is shared.  At the second time we
> > > > > > >   associate this entry, we create this rb-tree and store its root in
> > > > > > >   page->private(not used in fsdax).  Insert (->mapping, ->index) when
> > > > > > >   dax_associate_entry() and delete it when dax_disassociate_entry().
> > > > > > 
> > > > > > Do we really want to track all of this on a per-page basis?  I would
> > > > > > have thought a per-extent basis was more useful.  Essentially, create
> > > > > > a new address_space for each shared extent.  Per page just seems like
> > > > > > a huge overhead.
> > > > > > 
> > > > > Per-extent tracking is a nice idea for me.  I haven't thought of it
> > > > > yet...
> > > > > 
> > > > > But the extent info is maintained by filesystem.  I think we need a way
> > > > > to obtain this info from FS when associating a page.  May be a bit
> > > > > complicated.  Let me think about it...
> > > > 
> > > > That's why I want the -user of this association- to do a filesystem
> > > > callout instead of keeping it's own naive tracking infrastructure.
> > > > The filesystem can do an efficient, on-demand reverse mapping lookup
> > > > from it's own extent tracking infrastructure, and there's zero
> > > > runtime overhead when there are no errors present.
> > > > 
> > > > At the moment, this "dax association" is used to "report" a storage
> > > > media error directly to userspace. I say "report" because what it
> > > > does is kill userspace processes dead. The storage media error
> > > > actually needs to be reported to the owner of the storage media,
> > > > which in the case of FS-DAX is the filesytem.
> > > 
> > > Understood.
> > > 
> > > BTW, this is the usage in memory-failure, so what about rmap?  I have not
> > > found how to use this tracking in rmap.  Do you have any ideas?
> > > 
> > > > 
> > > > That way the filesystem can then look up all the owners of that bad
> > > > media range (i.e. the filesystem block it corresponds to) and take
> > > > appropriate action. e.g.
> > > 
> > > I tried writing a function to look up all the owners' info of one block in
> > > xfs for memory-failure use.  It was dropped in this patchset because I found
> > > out that this lookup function needs 'rmapbt' to be enabled when mkfs.  But
> > > by default, rmapbt is disabled.  I am not sure if it matters...
> > 
> > I'm pretty sure you can't have shared extents on an XFS filesystem if you
> > _don't_ have the rmapbt feature enabled.  I mean, that's why it exists.
> 
> You're confusing reflink with rmap. :)
> 
> rmapbt does all the reverse mapping tracking, reflink just does the
> shared data extent tracking.
> 
> But given that anyone who wants to use DAX with reflink is going to
> have to mkfs their filesystem anyway (to turn on reflink) requiring
> that rmapbt is also turned on is not a big deal. Especially as we
> can check it at mount time in the kernel...

Are we going to turn on rmap by default?  The last I checked, it did
have a 10-20% performance cost on extreme metadata-heavy workloads.
Or do we only enable it by default if mkfs detects a pmem device?

(Admittedly, most people do not run fsx as a productivity app; the
normal hit is usually 3-5% which might not be such a big deal since you
also get (half of) online fsck. :P)

--D

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx