Re: [PATCH 20/20] honor r/w changes at do_remount() time

[Herbert Poetzl <herbert@13thfloor.at>]

On Wed, Jun 28, 2006 at 06:19:35AM +0100, Al Viro wrote:
> On Tue, Jun 27, 2006 at 03:14:57PM -0700, Dave Hansen wrote:
> > 
> > Originally from: Herbert Poetzl <herbert@13thfloor.at>
> > 
> > This is the core of the read-only bind mount patch set.
> > 
> > Note that this does _not_ add a "ro" option directly to
> > the bind mount operation.  If you require such a mount,
> > you must first do the bind, then follow it up with a
> > 'mount -o remount,ro' operation.
>  
> I guess the fundamental problem I have with that approach is that it's
> a cop-out - we just declare rw state of vfsmount independent from that
> of filesystem and add a "if a flag is set, act upon vfsmount".

IMHO the read only check has to be done twice, i.e
once for the superblock and a second time for the
particular vfs mount, similar, the procfs mounts
entry shows the combination (logical and) of the
write ability ...

> And yes, some of that does make sense. Fine, let's separate that
> stuff; but then we'd better decide what rw superblock *is*.

> We have a number of vfsmounts over given superblock. OK, some are "we
> don't even ask them to be r/w". Some are "we want them r/w, but don't
> actually use as such at the moment". Some are "pinned down for write
> now". And we do get logics for "can't make it r/o right now".
>
> But look - we have the _same_ logics for superblock itself. Only it's
> full of holes. And since you have rw states for those completely
> unrelated to those of vfsmount, we get a ridiculous situation - we
> *do* mark the moments when superblock becomes impossible to remount
> r/o and we even mostly get the moments when it ceases to be busy
> writing (unlinked-but-opened files are major exception). But we can't
> use that information.
>
> So "can we remount superblock ro?" turns into kinda-sorta duplicate of
> the same for vfsmounts, but it's racy as hell and bloody incomplete;
> we don't even get "if some vfsmount over it is busy writing, we won't
> remount r/o". Approximation is done, but that's it. E.g. mkdir() in
> progress does *not* stop remount of superblock r/o (it does prevent
> remount of vfsmount with your patchset).
>
> FWIW, I suspect that the root of the problem is that we confuse
> different states of filesystem. E.g. one obviously useful feature
> would be to have soft r/o - filesystem that is (from the driver POV)
> mounted readonly, but would get transparently switched r/w at the
> first request. And you have all vfsmount-side infrastructure for that,
> BTW. Add something like mechanism we use for expiry and you've got
> a very tasty feature for e.g. laptop users: e.g. userland asking to
> switch fs soft-ro every 15 minutes and if nobody had wanted it r/w
> since the last time, do the transition; if asked r/w again, r/w it
> goes on its own. IOW, there's more to it than one bit. And I'm talking
> about superblock state...

in what way would that help laptop users?

the only case I see (where it could help) is if the
laptop runs out of battery and the filesystems are
in clean state, they will not ahve to do a filesystem
check when they replaced the battery, what am I 
obviously missing here?

TIA,
Herbert

> BTW, it might be worth doing the following:
> 	* reintroduce the list of vfsmounts over given superblock 
>         (protected by vfsmount_lock)
> 	* keep ro flag separate from counter and split it in two.
> 	* all decrements are with atomic_dec_and_lock()
> 	* all increments are with atomic_add_unless() + spin_lock() +
> 	  check flags + atomic_add_return() + possible spin_unlock
> 	* if writers count goes from non-zero to zero or vice versa
>         increment/decrement superblock counter (number of 
>         vfsmounts that really want write access).
> 	* make the moments when i_nlink hits 0 bump the superblock 
>         writers count; drop it when such sucker gets freed on final iput.
> 	* kill the sodding "traverse the list of opened files" 
>         logics in remounting superblock r/o.  Instead of that, 
>         grab spinlock, check writers count, bail out if non-zero, 
>	  grab vfsmount_lock, traverse the list over superblock and 
>	  set one of the flags, drop the locks and proceed.
> 	* when remounting superblock r/w, traverse the list and 
>	  knock out the same flag.
> 
> At least that way we'd get the majority of "can remount ro" logics right...
> 
> Another fun issues:
> 	a) MS_REC handling with MS_BIND remounts (trivial)
> 	b) figuring out what (if anything) should be done with 
>	   propagation when we have shared subtrees... (not trivial at all)
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