Re: [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

[Boris Brezillon]

On Tue, 29 Aug 2023 11:44:13 +0200
Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx> wrote:

> On Tue, 29 Aug 2023 10:52:03 +0200
> Christian König <christian.koenig@xxxxxxx> wrote:
> 
> > Am 29.08.23 um 09:29 schrieb Boris Brezillon:  
> > > On Tue, 29 Aug 2023 05:34:23 +0300
> > > Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> > >    
> > >> On 8/28/23 13:12, Boris Brezillon wrote:    
> > >>> On Sun, 27 Aug 2023 20:54:43 +0300
> > >>> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> > >>>        
> > >>>> In a preparation of adding drm-shmem memory shrinker, move all reservation
> > >>>> locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that
> > >>>> will resolve spurious lockdep warning about wrong locking order vs
> > >>>> fs_reclam code paths during freeing of shmem GEM, where lockdep isn't
> > >>>> aware that it's impossible to have locking contention with the fs_reclam
> > >>>> at this special time.
> > >>>>
> > >>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> > >>>> ---
> > >>>>   drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++---------
> > >>>>   1 file changed, 25 insertions(+), 12 deletions(-)
> > >>>>
> > >>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > >>>> index d96fee3d6166..ca5da976aafa 100644
> > >>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > >>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > >>>> @@ -128,6 +128,23 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
> > >>>>   }
> > >>>>   EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
> > >>>>   
> > >>>> +static void drm_gem_shmem_resv_assert_held(struct drm_gem_shmem_object *shmem)
> > >>>> +{
> > >>>> +	/*
> > >>>> +	 * Destroying the object is a special case.. drm_gem_shmem_free()
> > >>>> +	 * calls many things that WARN_ON if the obj lock is not held.  But
> > >>>> +	 * acquiring the obj lock in drm_gem_shmem_free() 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, we pretend it is already locked.
> > >>>> +	 */
> > >>>> +	if (kref_read(&shmem->base.refcount))
> > >>>> +		drm_gem_shmem_resv_assert_held(shmem);
> > >>>> +}
> > >>>> +
> > >>>>   /**
> > >>>>    * drm_gem_shmem_free - Free resources associated with a shmem GEM object
> > >>>>    * @shmem: shmem GEM object to free
> > >>>> @@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> > >>>>   	if (obj->import_attach) {
> > >>>>   		drm_prime_gem_destroy(obj, shmem->sgt);
> > >>>>   	} else if (!shmem->imported_sgt) {
> > >>>> -		dma_resv_lock(shmem->base.resv, NULL);
> > >>>> -
> > >>>>   		drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
> > >>>>   
> > >>>>   		if (shmem->sgt) {
> > >>>> @@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> > >>>>   			drm_gem_shmem_put_pages_locked(shmem);    
> > >>> AFAICT, drm_gem_shmem_put_pages_locked() is the only function that's
> > >>> called in the free path and would complain about resv-lock not being
> > >>> held. I think I'd feel more comfortable if we were adding a
> > >>> drm_gem_shmem_free_pages() function that did everything
> > >>> drm_gem_shmem_put_pages_locked() does except for the lock_held() check
> > >>> and the refcount dec, and have it called here (and in
> > >>> drm_gem_shmem_put_pages_locked()). This way we can keep using
> > >>> dma_resv_assert_held() instead of having our own variant.    
> > >> It's not only drm_gem_shmem_free_pages(), but any drm-shmem function
> > >> that drivers may use in the GEM's freeing callback.
> > >>
> > >> For example, panfrost_gem_free_object() may unpin shmem BO and then do
> > >> drm_gem_shmem_free().    
> > > Is this really a valid use case?    
> > 
> > I haven't followed the whole discussion, but I think it isn't a valid 
> > use case.
> > 
> > That page_use_count is none zero while the GEM object is about to be 
> > destroyed can only happen is someone managed to grab a reference to the 
> > page without referencing the GEM object.  
> 
> Actually, drm_gem_shmem_object is a bit special (weird?) in this regard.
> drm_gem_shmem_get_pages_sgt_locked() creates the sgt and takes a
> pages ref (pages_use_count++). The sgt itself is cached (next call to
> drm_gem_shmem_get_pages_sgt_locked() will return the existing sgt) but
> not refcounted, which means it will stay around until the GEM object is
> destroyed or its pages are purged (GEM eviction). Because of that,
> shmem->pages_use_count == 1 in drm_gem_shmem_free_pages() is valid iff
> shmem->sgt != NULL. pages_use_count > 1 is invalid though, as should be
> pages_pin_count after Dmitry's patches.
> 
> If we want to 'fix' that (not convinced this is a bug, more a design
> choice), we need to refcount the sgt users and add
> drm_gem_shmem_put_pages_sgt[_locked](), so drivers can reflect when
> they're done using the sgt.

Or we simply create the sgt in drm_gem_shmem_get_pages_locked(), and
make drm_gem_shmem_get_pages_sgt() a dummy wrapper returning
shmem->sgt, which will force callers to explicitly call
drm_gem_shmem_{get,pin}_pages[_locked]() if they want a non-NULL sgt.
By doing that, we avoid adding yet another level of refcounting and we
keep drivers responsible for pages_{use,pin}_count balancing. The only
downside would be the unconditional creation of the sg_table, but I
suspect all current users of drm_gem_shmem_object want this sg_table
anyway.