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SubjectRe: [PATCH] mm: cleancache: fix potential race in cleancache apis
On Thu, Jul 01, 2021 at 02:06:44PM +0900, 권오훈 wrote:
> On Thu, Jul 1, 2021 at 02:06:45PM +0900, 권오훈 wrote:
> > 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
> >
> Dear all.
>
> We are using Cleancache APIs for our proprietary feature in Samsung.
> As Wilcox mentioned, however, there is no cleancache backend in current kernel
> mainline.
> So if the race patch shall be accepted, then it seems unnecessary to patch
> previous stable kernels.
>
> Meanwhile, I personally think cleancache API still has potential to be a good
> material when used with new arising technologies such as pmem or NVMe.
>
> So I suggest to postpone removing cleancache for a while.

If there are no in-kernel users, it needs to be removed. If you rely on
this, wonderful, please submit your code as soon as possible.

thanks,

greg k-h

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