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

[Darrick J. Wong]

On Thu, Jun 04, 2020 at 03:37:42PM +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.
> 
> Hi Dave,
> 
> I ran into some difficulties when trying to implement the per-extent rmap
> tracking.  So, I re-read your comments and found that I was misunderstanding
> what you described here.
> 
> I think what you mean is: we don't need the in-memory dax-rmap tracking now.
> Just ask the FS for the owner's information that associate with one page
> when memory-failure.  So, the per-page (even per-extent) dax-rmap is
> needless in this case.  Is this right?

Right.  XFS already has its own rmap tree.

> Based on this, we only need to store the extent information of a fsdax page
> in its ->mapping (by searching from FS).  Then obtain the owners of this
> page (also by searching from FS) when memory-failure or other rmap case
> occurs.

I don't even think you need that much.  All you need is the "physical"
offset of that page within the pmem device (e.g. 'this is the 307th 4k
page == offset 1257472 since the start of /dev/pmem0') and xfs can look
up the owner of that range of physical storage and deal with it as
needed.

> So, a fsdax page is no longer associated with a specific file, but with a
> FS(or the pmem device).  I think it's easier to understand and implement.

Yes.  I also suspect this will be necessary to support reflink...

--D

> 
> --
> Thanks,
> Ruan Shiyang.
> > 
> > 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.
> > 
> > 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.
> > 
> > - if it falls in filesytem metadata, shutdown the filesystem
> > - if it falls in user data, call the "kill userspace dead" routines
> >    for each mapping/index tuple the filesystem finds for the given
> >    LBA address that the media error occurred.
> > 
> > Right now if the media error is in filesystem metadata, the
> > filesystem isn't even told about it. The filesystem can't even shut
> > down - the error is just dropped on the floor and it won't be until
> > the filesystem next tries to reference that metadata that we notice
> > there is an issue.
> > 
> > Cheers,
> > 
> > Dave.
> > 
> 
>