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
