Re: [PATCH v3 3/5] mm: Notify filesystems when it's time to apply adeferred cmtime update

[Jan Kara]

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.

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.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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