Messages in this thread | | | Date | Wed, 28 Jun 2006 06:19:35 +0100 | From | Al Viro <> | Subject | Re: [PATCH 20/20] honor r/w changes at do_remount() time |
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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".
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...
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) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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