possible deadlock in shmem_fallocate (4)

[Michal Hocko]

On Tue 14-07-20 13:32:05, Hillf Danton wrote:
> 
> On Mon, 13 Jul 2020 20:41:11 -0700 Eric Biggers wrote:
> > On Tue, Jul 14, 2020 at 11:32:52AM +0800, Hillf Danton wrote:
> > > 
> > > Add FALLOC_FL_NOBLOCK and on the shmem side try to lock inode upon the
> > > new flag. And the overall upside is to keep the current gfp either in
> > > the khugepaged context or not.
> > > 
> > > --- a/include/uapi/linux/falloc.h
> > > +++ b/include/uapi/linux/falloc.h
> > > @@ -77,4 +77,6 @@
> > >   */
> > >  #define FALLOC_FL_UNSHARE_RANGE		0x40
> > >  
> > > +#define FALLOC_FL_NOBLOCK		0x80
> > > +
> > 
> > You can't add a new UAPI flag to fix a kernel-internal problem like this.
> 
> Sounds fair, see below.
> 
> What the report indicates is a missing PF_MEMALLOC_NOFS and it's
> checked on the ashmem side and added as an exception before going
> to filesystem. On shmem side, no more than a best effort is paid
> on the inteded exception.
> 
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -437,6 +437,7 @@ static unsigned long
>  ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  {
>  	unsigned long freed = 0;
> +	bool nofs;
>  
>  	/* We might recurse into filesystem code, so bail out if necessary */
>  	if (!(sc->gfp_mask & __GFP_FS))
> @@ -445,6 +446,11 @@ ashmem_shrink_scan(struct shrinker *shri
>  	if (!mutex_trylock(&ashmem_mutex))
>  		return -1;
>  
> +	/* enter filesystem with caution: nonblock on locking */
> +	nofs = current->flags & PF_MEMALLOC_NOFS;
> +	if (!nofs)
> +		current->flags |= PF_MEMALLOC_NOFS;
> +
>  	while (!list_empty(&ashmem_lru_list)) {
>  		struct ashmem_range *range =
>  			list_first_entry(&ashmem_lru_list, typeof(*range), lru);

I do not think this is an appropriate fix. First of all is this a real
deadlock or a lockdep false positive? Is it possible that ashmem just
needs to properly annotate its shmem inodes? Or is it possible that
the internal backing shmem file is visible to the userspace so the write
path would be possible?

If this a real problem then the proper fix would be to set internal
shmem mapping's gfp_mask to drop __GFP_FS.
-- 
Michal Hocko
SUSE Labs