On Thu, Jun 04, 2020 at 03:37:42PM +0800, Ruan Shiyang wrote:
On 2020/4/28 下午2:43, Dave Chinner wrote:
> On Tue, Apr 28, 2020 at 06:09:47AM +0000, Ruan, Shiyang wrote:
> > 在 2020/4/27 20:28:36, "Matthew Wilcox" <willy(a)infradead.org>
> > > On Mon, Apr 27, 2020 at 04:47:42PM +0800, Shiyang Ruan wrote:
> > > > This patchset is a try to resolve the shared 'page cache'
> > > > 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
> > > > will be associated more than once if is shared. At the second time
> > > > associate this entry, we create this rb-tree and store its root in
> > > > page->private(not used in fsdax). Insert (->mapping,
> > > > 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.
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
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
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
So, a fsdax page is no longer associated with a specific file, but
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...
> Ruan Shiyang.
> > At the moment, this "dax association" is used to "report" a
> > 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.