On Tue, Aug 20, 2013 at 05:47:10PM -0700, Andy Lutomirski wrote: > On Tue, Aug 20, 2013 at 3:43 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Tue, Aug 20, 2013 at 02:54:01PM -0700, Andy Lutomirski wrote: > >> On Tue, Aug 20, 2013 at 2:48 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> > On Tue, Aug 20, 2013 at 09:42:34AM -0700, Andy Lutomirski wrote: > >> >> On Tue, Aug 20, 2013 at 9:00 AM, Jan Kara <jack@xxxxxxx> wrote: > >> >> > On Mon 19-08-13 21:14:44, Andy Lutomirski wrote: > >> >> >> >> I could require ->writepages *and* ->flush_cmtime to handle the time > >> >> >> >> update, but that would complicate non-transactional filesystems. > >> >> >> >> Those filesystems should just flush cmtime at the end of writepages. > >> >> >> > > >> >> >> > do_writepages() is the wrong place to do such updates - we can get > >> >> >> > writeback directly through .writepage, so the time updates need to > >> >> >> > be in .writepage. That first .writepage call will clear the bit on > >> >> >> > the mapping, so it's only done on the first call to .writepage on > >> >> >> > the given mapping. > >> >> >> > >> >> >> Last time I checked, all the paths that actually needed the timestamp > >> >> >> update went through .writepages. I'll double-check. > >> >> > kswapd can call just .writepage to do the writeout so timestamp update > >> >> > should be handled there as well. Otherwise all pages in a mapping can be > >> >> > cleaned without timestamp being updated. > >> >> > >> >> OK, I'll fix that. > >> >> > >> >> > > >> >> > Which btw made me realize that even your scheme doesn't completely make > >> >> > sure timestamp is updated after mmap write - if you have pages 0 and 1, you > >> >> > write to both of them - CMTIME flag gets set. Then fsync_range(fd, 0, 4096) > >> >> > is called. We write the page 0, writeprotect it, update timestamps. But > >> >> > page 1 is still writeable so writes to it won't set CMTIME flag, neither > >> >> > update the timestamp... Not that I think this can be reasonably solved but > >> >> > it is a food for thought. > >> >> > >> >> This should already work. AS_CMTIME is set when the pte goes from > >> >> dirty to clean, not when the pte goes from wp to writable. So > >> >> whenever clear_page_dirty_for_io is called on page 1, AS_CMTIME will > >> >> be set and a subsequent writepages call will update the timestamp. > >> > > >> > Oh, I missed that - I thought you were setting AS_CMTIME during > >> > .page_mkwrite. > >> > > >> > Setting it in clear_page_dirty_for_io() is too late for filesystems > >> > to include it in their existing transactions during .writepage, (at > >> > least for XFs and ext4) because they do their delayed allocation > >> > transactions before changing page state.... > >> > >> Couldn't it go between mpage_map_and_submit_extent and > >> ext4_journal_stop in ext4_writepages? > > > > Maybe - I'm not an ext4 expert - but even if you can make it work > > for ext4 in some way, that doesn't mean it is possible for any other > > filesystem to use the same method. You're adding code to generic, > > non-filesystem specific code paths and so the solutions need to be > > generic rather not tied to how a specific filesystem is implemented. > > > > I don't see the problem for xfs or btrfs either. > > xfs uses generic_writepages, which already does the right thing. (xfs > with my updated patches passes my tests.) xfs_vm_writepage calls > xfs_start_page_writeback(..., 1, ...), so clear_page_dirty_for_io is > called. At that point (I presume), it would still be possible to add > metadata to a transaction (assuming there's a transaction open -- I > don't have a clue here). That's my point - there isn't a transaction in XFS at this point, and so if we gather that flag from clear_page_dirty_for_io() we'd need a second transaction and therefore the optimisation you want filesystems to use to mitigate the additional overhead can't be done for all commonly used filesystems. > Even if this is too late, xfs_vm_writepage > could call page_mkwrite to for AS_CMTIME to be set if needed. > page_mkwrite will be fast if the page isn't mmapped. What am I > missing? That it leads to different behaviour for different filesystems. i.e. page_mkwrite on page A sets the flag. writeback on a range that doesn't include page A occurs, sees the flag, clears it after updating the timestamp. Some time later writeback on page A occurs, no timestamp update occurs. The behaviour needs to be consistent across filesystems. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
