Ceph and Netfslib

[David Howells <dhowells@redhat.com>]

Hi Alex, Slava,

I don't know whether you know, but I'm working on netfslib-ising ceph with an
eye to moving all the VFS/VM normal I/O interfaces to netfslib (->read_iter,
->write_iter, ->readahead, ->read_folio, ->page_mkwrite, ->writepages), though
with wrapping/methods by which each network filesystem can add its own
distinctive flavour.

Also, that would include doing things like content encryption, since that is
generally useful in filesystems and I have plans to support it in both AFS and
CIFS as well.

This means that fs/ceph/ will have practically nothing to do with page structs
or folio structs.  All that will be offloaded to netfslib and netfslib will
just hand iov_iters to the client filesystems, including ceph.

This will also allow me to massively simplify the networking code in
net/ceph/.  My aim is to replace all the page array, page lists, bio,
etc. data types in libceph with a single type that just conveys an iov_iter
and I have a ceph_databuf type that holds a list of pages in the form of a
bio_vec[] and I can extract an iov_iter from that to pass to the networking.

Then, for the transmission side, the iov_iter will be passed to the TCP socket
with MSG_SPLICE_PAGES rather than iterating over the data type and passing a
page fragment at a time.  We fixed this up for nfsd and Chuck Lever reported a
improvement in throughput (15% if I remember correctly).

The patches I have so far can be found here:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=ceph-iter

Note that I have rbd working with the changes I've made to that point.


Anyway, ... I need to pick someone's brain about whether the way per-page
tracking of snapshots within fs/ceph/ can be simplified.

Firstly, note that there may be a bug in ceph writeback cleanup as it stands.
It calls folio_detach_private() without holding the folio lock (it holds the
writeback lock, but that's not sufficient by MM rules).  This means you have a
race between { setting ->private, setting PG_private and inc refcount } on one
hand and { clearing ->private, clearing PG_private and dec refcount } on the
other.

Unfortunately, you cannot just take the page lock from writeback cleanup
without running the risk of deadlocking against ->writepages() wanting to take
PG_lock and then PG_writeback.  And you cannot drop PG_writeback first as the
moment you do that, the page can be deallocated.


Secondly, there's a counter, ci->i_wrbuffer_ref, that might actually be
redundant if we do it right as I_PINNING_NETFS_WB offers an alternative way we
might do things.  If we set this bit, ->write_inode() will be called with
wbc->unpinned_netfs_wb set when all currently dirty pages have been cleaned up
(see netfs_unpin_writeback()).  netfslib currently uses this to pin the
fscache objects but it could perhaps also be used to pin the writeback cap for
ceph.


Thirdly, I was under the impression that, for any given page/folio, only the
head snapshot could be altered - and that any older snapshot must be flushed
before we could allow that.


Fourthly, the ceph_snap_context struct holds a list of snaps.  Does it really
need to, or is just the most recent snap for which the folio holds changes
sufficient?

Thanks,
David