On Tue, Apr 28, 2020 at 05:32:41PM +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.
> 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.
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.