[PATCH v16 15/20] drm/shmem-helper: Add memory shrinker

[Boris Brezillon]

On Thu, 14 Sep 2023 07:02:52 +0300
Dmitry Osipenko <dmitry.osipenko at collabora.com> wrote:

> On 9/13/23 10:48, Boris Brezillon wrote:
> > On Wed, 13 Sep 2023 03:56:14 +0300
> > Dmitry Osipenko <dmitry.osipenko at collabora.com> wrote:
> >   
> >> On 9/5/23 11:03, Boris Brezillon wrote:  
> >>>>                * But
> >>>> +		 * acquiring the obj lock in drm_gem_shmem_release_pages_locked() can
> >>>> +		 * cause a locking order inversion between reservation_ww_class_mutex
> >>>> +		 * and fs_reclaim.
> >>>> +		 *
> >>>> +		 * This deadlock is not actually possible, because no one should
> >>>> +		 * be already holding the lock when drm_gem_shmem_free() is called.
> >>>> +		 * Unfortunately lockdep is not aware of this detail.  So when the
> >>>> +		 * refcount drops to zero, don't touch the reservation lock.
> >>>> +		 */
> >>>> +		if (shmem->got_pages_sgt &&
> >>>> +		    refcount_dec_and_test(&shmem->pages_use_count)) {
> >>>> +			drm_gem_shmem_do_release_pages_locked(shmem);
> >>>> +			shmem->got_pages_sgt = false;
> >>>>  		}    
> >>> Leaking memory is the right thing to do if pages_use_count > 1 (it's
> >>> better to leak than having someone access memory it no longer owns), but
> >>> I think it's worth mentioning in the above comment.    
> >>
> >> It's unlikely that it will be only a leak without a following up
> >> use-after-free. Neither is acceptable.  
> > 
> > Not necessarily, if you have a page leak, it could be that the GPU has
> > access to those pages, but doesn't need the GEM object anymore
> > (pages are mapped by the iommu, which doesn't need shmem->sgt or
> > shmem->pages after the mapping is created). Without a WARN_ON(), this
> > can go unnoticed and lead to memory corruptions/information leaks.
> >   
> >>
> >> The drm_gem_shmem_free() could be changed such that kernel won't blow up
> >> on a refcnt bug, but that's not worthwhile doing because drivers
> >> shouldn't have silly bugs.  
> > 
> > We definitely don't want to fix that, but we want to complain loudly
> > (WARN_ON()), and make sure the risk is limited (preventing memory from
> > being re-assigned to someone else by not freeing it).  
> 
> That's what the code did and continues to do here. Not exactly sure what
> you're trying to say. I'm going to relocate the comment in v17 to
> put_pages(), we can continue discussing it there if I'm missing yours point.
> 

I'm just saying it would be worth mentioning that we're intentionally
leaking memory if shmem->pages_use_count > 1. Something like:

	/**
	 * shmem->pages_use_count should be 1 when ->sgt != NULL and
	 * zero otherwise. If some users still hold a pages reference
	 * that's a bug, and we intentionally leak the pages so they
	 * can't be re-allocated to someone else while the GPU/CPU
	 * still have access to it.
	 */
	drm_WARN_ON(drm,
		    refcount_read(&shmem->pages_use_count) == (shmem->sgt ? 1 : 0));
	if (shmem->sgt && refcount_dec_and_test(&shmem->pages_use_count))
		drm_gem_shmem_free_pages(shmem);