Re: [PATCH] mm: cleancache: fix potential race in cleancache apis

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Wed, Jun 30, 2021 at 12:26:57PM +0100, Matthew Wilcox wrote:
> On Wed, Jun 30, 2021 at 10:13:28AM +0200, gregkh@linuxfoundation.org wrote:
> > On Wed, Jun 30, 2021 at 04:33:10PM +0900, 권오훈 wrote:
> > > Current cleancache api implementation has potential race as follows,
> > > which might lead to corruption in filesystems using cleancache.
> > > 
> > > thread 0                thread 1                        thread 2
> > > 
> > >                         in put_page
> > >                         get pool_id K for fs1
> > > invalidate_fs on fs1
> > > frees pool_id K
> > >                                                         init_fs for fs2
> > >                                                         allocates pool_id K
> > >                         put_page puts page
> > >                         which belongs to fs1
> > >                         into cleancache pool for fs2
> > > 
> > > At this point, a file cache which originally belongs to fs1 might be
> > > copied back to cleancache pool of fs2, which might be later used as if
> > > it were normal cleancache of fs2, and could eventually corrupt fs2 when
> > > flushed back.
> > > 
> > > Add rwlock in order to synchronize invalidate_fs with other cleancache
> > > operations.
> > > 
> > > In normal situations where filesystems are not frequently mounted or
> > > unmounted, there will be little performance impact since
> > > read_lock/read_unlock apis are used.
> > > 
> > > Signed-off-by: Ohhoon Kwon <ohoono.kwon@samsung.com>
> > 
> > What commit does this fix?  Should it go to stable kernels?
> 
> I have a commit I haven't submitted yet with this changelog:
> 
>     Remove cleancache
> 
>     The last cleancache backend was deleted in v5.3 ("xen: remove tmem
>     driver"), so it has been unused since.  Remove all its filesystem hooks.
> 
>     Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

That's even better!

But if so, how is the above reported problem even a problem if no one is
using cleancache?

thanks,

greg k-h