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

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Date:   Thu, 14 Sep 2023 09:36:26 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v16 15/20] drm/shmem-helper: Add memory shrinker

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

> On 9/13/23 10:48, Boris Brezillon wrote:
> > On Wed, 13 Sep 2023 03:56:14 +0300
> > Dmitry Osipenko <dmitry.osipenko@...labora.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);