Re: [PATCH, RFC] check for frozen filesystems in the mmap path

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> KOSAKI Motohiro wrote:
> >>> 2. this logic kill multi thread application.
> >>>
> >>> this logic mean mmap_sem grabbing until unfreeze.
> >>> it mean othrer thread in the same process can't page-fault although
> >>> it don't touch frozen-sb.
> >>> it seems strange.
> >> Hm, I hadn't thought about this ... On the one hand, ->page_mkwrite can
> >> already sleep, though a userspace freeze/unfreeze could potentially take
> >> much much longer.  freeze/unfreeze *should* happen very quickly, but
> >> nothing enforces that.
> >>
> >> Do you have any suggestions?
> > 
> > One more comment.
> > 
> > I read ioctl_fsfreeze() and freeze_bdev(), it call __fsync_super().
> > Oh, I don't think __fsync_suepr is very quick.
> 
> Well, what I mean is that the filesystem is not intended to be frozen
> for long periods of time.  But it's not enforced by any method.
> 
> > So, page-fault have one unique characteristics.
> > if page-fault return 0 without pte change, page-fault is occur again soon.
> > then, if you need long time waiting, I think you can use following technique.
> > 
> > 	unlock mmap_sem
> > 	wait long-time
> > 	lock mmap_sem
> > 	goto out;
> > 
> > 
> > it cause page-fault counter increment twice unintesionally.
> > but no problem. fs-freeze is not freqently event.
> > 
> > Am I missing anything?
> 
> Hm, I'll have to think about that.  This is not my best area.  :)  So do
> you mean that if a wait needs to happen for the frozen fs, we can
> unlock, do that wait for unfreeze, relock, return early, and come back
> again when it is not frozen?

Yes.


> One other thing that I think I just discovered is that nothing is
> actually stopping mmap IO even on a frozen filesystem, as long as no
> metadata updates are required for the IO... I'm seeing this on xfs
> anyway (ext4 tries to update mtime, so that gets stopped on the frozen fs).

I don't understand this issue. oh sorry, I'm not fs expert ;)