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

[Ruan Shiyang]

On 2020/6/5 äå9:30, Dave Chinner wrote:
> On Thu, Jun 04, 2020 at 07:51:07AM -0700, Darrick J. Wong wrote:
> > 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.
>
> *nod*
>
> > > 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.
>
> Right. If we have the dax device associated with the page that had
> the failure, then we can determine the offset of the page into the
> block device address space and that's all we need to find the owner
> of the page in the filesystem.
>
> Note that there may actually be no owner - the page that had the
> fault might land in free space, in which case we can simply zero
> the page and clear the error.

OK.  Thanks for pointing out.

> > > 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.
>
> Effectively, yes. But we shouldn't need to actually associate the
> page with anything at the filesystem level because it is already
> associated with a DAX device at a lower level via a dev_pagemap.
> The hardware page fault already runs thought this code
> memory_failure_dev_pagemap() before it gets to the DAX code, so
> really all we need to is have that function pass us the page, offset
> into the device and, say, the struct dax_device associated with that
> page so we can get to the filesystem superblock we can then use for
> rmap lookups on...

OK.  I was just thinking about how can I execute the FS rmap search from 
the memory-failure.  Thanks again for pointing out. :)


--
Thanks,
Ruan Shiyang.

> Cheers,
>
> Dave.