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

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 14 Sep 2023 14:36:23 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> On 9/14/23 11:27, Boris Brezillon wrote:
> > On Thu, 14 Sep 2023 10:50:32 +0300
> > Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> >   
> >> On 9/14/23 10:36, Boris Brezillon wrote:  
> >>> On Thu, 14 Sep 2023 07:02:52 +0300
> >>> Dmitry Osipenko <dmitry.osipenko@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@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);    
> >>
> >> That may be acceptable, but only once there will a driver using this
> >> feature.  
> > 
> > Which feature? That's not related to a specific feature, that's just
> > how drm_gem_shmem_get_pages_sgt() works, it takes a pages ref that can
> > only be released in drm_gem_shmem_free(), because sgt users are not
> > refcounted and the sgt stays around until the GEM object is freed or
> > its pages are evicted. The only valid cases we have at the moment are:
> > 
> > - pages_use_count == 1 && sgt != NULL
> > - pages_use_count == 0
> > 
> > any other situations are buggy.  
> 
> sgt may belong to dma-buf for which pages_use_count=0, this can't be
> done until sgt mess is sorted out

No it can't, not in that path, because the code you're adding is in the
if (!obj->import_branch) branch:


 	if (obj->import_attach) {
 		drm_prime_gem_destroy(obj, shmem->sgt);
 	} else {
		...
		// Your changes are here.
		...
	}
>