Re: (un)loadable module support for zcache

[Andor Daam <andor.daam@googlemail.com>]

2012/3/8 Dan Magenheimer <dan.magenheimer@oracle.com>
>
> > From: Florian Schmaus [mailto:fschmaus@gmail.com]
> > Subject: Re: (un)loadable module support for zcache
> >
> > On 03/05/12 17:57, Dan Magenheimer wrote:
> > > I think the answer here is for cleancache (and frontswap) to
> > > support "lazy pool creation".  If a backend has not yet
> > > registered when an init_fs/init call is made, cleancache
> > > (or frontswap) must record the attempt and generate a valid
> > > "fake poolid" to return.  Any calls to put/get/flush with
> > > a fake poolid is ignored as the zcache module is not
> > > yet loaded.  Later, when zcache is insmod'ed, it will attempt
> > > to register and cleancache must then call the init_fs/init
> > > routines (to "lazily" create the pools), obtain a "real poolid"
> > > from zcache for each pool and "map" the fake poolid to the real
> > > poolid on EVERY get/put/flush and on pool destroy (umount/swapoff).
> >
> > We were thinking about how to make cleancache and frontswap able to cope
> > with the mounting of filesystems and running of swapon when there is no
> > backend registered without adding an indirection caused by a fake pool
> > id map.
> >
> > We figured a way to deal with this in cleancache would be to store the
> > struct super_block pointers in an array for every call to init_fs and
> > the uuids and struct super_blocks pointers in different arrays for every
> > call to init_shared_fs. When a filesystem unmounts before a backend is
> > registered, its entries in the respective arrays are removed.
> > While no backend is registered, the put_page() and invalidate_page() are
> > ignored and get_page() fails. As soon as a backend registers the init_fs
> > and init_shared_fs functions are called for the struct super_block
> > pointers (and uuids) stored in the according arrays.
> >
> > For frontswap we are aiming for a similar approach by remembering the
> > types for every call to init and failing put_page() and ignoring
> > get_page() and invalidate_page().
> > Again, when a backend registers init is called for every type stored.
> >
> > This should allow backends to register with cleancache and frontswap
> > even after the mounting of filesystems and/or swapon is run. Therefore
> > it should allow zcache to be insmodded. This would be a first step to
> > allow rmmodding of zcache aswell.
> >
> > Is this approach feasible?
>
> Hi Stefan, Florian, and Andor --
>
> I do see a potential problem with this approach.  You would
> be saving a superblock pointer and then using it later.  What
> if the filesystem was unmounted in the meantime?  Or, worse,
> what if it was unmounted and then the address of the superblock
> is reused to point to some completely different object?
>
> I think if you ensure that cleancache_invalidate_fs() is always
> called when a cleancache-enabled filesystem is unmounted,
> then in cleancache_invalidate_fs() you remove the matching
> superblock pointer from your arrays, then it should work.
>
> Dan

We already thought of removing the matching pointer, whenever a filesystem is
unmounted.
As the comment to __cleancache_invalidate_fs in cleancache.c states
that this function
is called by any cleancache-enabled filesystem at time of unmount, we
assumed that this function was actually always called upon unmount.
Is it not certain that this function is always called?

Andor

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