On Thu, 25 Jan 2024 16:47:24 +0000
Steven Price <steven.price@xxxxxxx> wrote:
> On 05/01/2024 18:46, Dmitry Osipenko wrote:
> > To simplify the drm-shmem refcnt handling, we're moving away from
> > the implicit get_pages() that is used by get_pages_sgt(). From now on
> > drivers will have to pin pages while they use sgt. Panfrost's shrinker
> > doesn't support swapping out BOs, hence pages are pinned and sgt is valid
> > as long as pages' use-count > 0.
> >
> > In Panfrost, panfrost_gem_mapping, which is the object representing a
> > GPU mapping of a BO, owns a pages ref. This guarantees that any BO being
> > mapped GPU side has its pages retained till the mapping is destroyed.
> >
> > Since pages are no longer guaranteed to stay pinned for the BO lifetime,
> > and MADVISE(DONT_NEED) flagging remains after the GEM handle has been
> > destroyed, we need to add an extra 'is_purgeable' check in
> > panfrost_gem_purge(), to make sure we're not trying to purge a BO that
> > already had its pages released.
> >
> > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>
> Reviewed-by: Steven Price <steven.price@xxxxxxx>
>
> Although I don't like the condition in panfrost_gem_mapping_release()
> for drm_gem_shmem_put_pages() and assigning NULL to bo->sgts - it feels
> very fragile. See below.
>
> > ---
> > drivers/gpu/drm/panfrost/panfrost_gem.c | 63 ++++++++++++++-----
> > .../gpu/drm/panfrost/panfrost_gem_shrinker.c | 6 ++
> > 2 files changed, 52 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> > index f268bd5c2884..7edfc12f7c1f 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> > @@ -35,20 +35,6 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
> > */
> > WARN_ON_ONCE(!list_empty(&bo->mappings.list));
> >
> > - if (bo->sgts) {
> > - int i;
> > - int n_sgt = bo->base.base.size / SZ_2M;
> > -
> > - for (i = 0; i < n_sgt; i++) {
> > - if (bo->sgts[i].sgl) {
> > - dma_unmap_sgtable(pfdev->dev, &bo->sgts[i],
> > - DMA_BIDIRECTIONAL, 0);
> > - sg_free_table(&bo->sgts[i]);
> > - }
> > - }
> > - kvfree(bo->sgts);
> > - }
> > -
> > drm_gem_shmem_free(&bo->base);
> > }
> >
> > @@ -85,11 +71,40 @@ panfrost_gem_teardown_mapping(struct panfrost_gem_mapping *mapping)
> >
> > static void panfrost_gem_mapping_release(struct kref *kref)
> > {
> > - struct panfrost_gem_mapping *mapping;
> > -
> > - mapping = container_of(kref, struct panfrost_gem_mapping, refcount);
> > + struct panfrost_gem_mapping *mapping =
> > + container_of(kref, struct panfrost_gem_mapping, refcount);
> > + struct panfrost_gem_object *bo = mapping->obj;
> > + struct panfrost_device *pfdev = bo->base.base.dev->dev_private;
> >
> > panfrost_gem_teardown_mapping(mapping);
> > +
> > + /* On heap BOs, release the sgts created in the fault handler path. */
> > + if (bo->sgts) {
> > + int i, n_sgt = bo->base.base.size / SZ_2M;
> > +
> > + for (i = 0; i < n_sgt; i++) {
> > + if (bo->sgts[i].sgl) {
> > + dma_unmap_sgtable(pfdev->dev, &bo->sgts[i],
> > + DMA_BIDIRECTIONAL, 0);
> > + sg_free_table(&bo->sgts[i]);
> > + }
> > + }
> > + kvfree(bo->sgts);
> > + }
> > +
> > + /* Pages ref is owned by the panfrost_gem_mapping object. We must
> > + * release our pages ref (if any), before releasing the object
> > + * ref.
> > + * Non-heap BOs acquired the pages at panfrost_gem_mapping creation
> > + * time, and heap BOs may have acquired pages if the fault handler
> > + * was called, in which case bo->sgts should be non-NULL.
> > + */
> > + if (!bo->base.base.import_attach && (!bo->is_heap || bo->sgts) &&
> > + bo->base.madv >= 0) {
> > + drm_gem_shmem_put_pages(&bo->base);
> > + bo->sgts = NULL;
>
> The assignment of NULL here really ought to be unconditional - it isn't
> a valid pointer because of the kvfree() above.
Fair enough. How about we drop the '|| bo->sgts' and add an
drm_gem_shmem_put_pages() to the above if (bo->sgts) block, where we'll
also assign bo->sgts to NULL?
>
> I also feel that the big condition above suggests there's a need for a
> better state machine to keep track of what's going on.
I'm planning to extend drm_gem_shmem to support the alloc-on-fault use
case that all Mali GPUs seem to rely on (lima, panfrost and soon
panthor would use those helpers). The idea is to:
- make the allocation non-blocking, so we can kill the blocking
allocation in the dma signalling path (basically what intel does)
- allow dynamic extension of the pages array using an xarray instead of
a plain array
Hopefully this makes the state tracking a lot easier, and we can also
get rid of the hack we have in panfrost/lima where we manipulate
drm_gem_shmem_object refcounts directly.
>
> But having said that I do think this series as a whole is an
> improvement, it's nice to get the shrinker code generic. And sadly I
> don't have an immediate idea for cleaning this up, hence my R-b.
>
> Steve
>
> > + }
> > +
> > drm_gem_object_put(&mapping->obj->base.base);
> > panfrost_mmu_ctx_put(mapping->mmu);
> > kfree(mapping);
> > @@ -125,6 +140,20 @@ int panfrost_gem_open(struct drm_gem_object *obj, struct drm_file *file_priv)
> > if (!mapping)
> > return -ENOMEM;
> >
> > + if (!bo->is_heap && !bo->base.base.import_attach) {
> > + /* Pages ref is owned by the panfrost_gem_mapping object.
> > + * For non-heap BOs, we request pages at mapping creation
> > + * time, such that the panfrost_mmu_map() call, further down in
> > + * this function, is guaranteed to have pages_use_count > 0
> > + * when drm_gem_shmem_get_pages_sgt() is called.
> > + */
> > + ret = drm_gem_shmem_get_pages(&bo->base);
> > + if (ret) {
> > + kfree(mapping);
> > + return ret;
> > + }
> > + }
> > +
> > INIT_LIST_HEAD(&mapping->node);
> > kref_init(&mapping->refcount);
> > drm_gem_object_get(obj);
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> > index 02b60ea1433a..d4fb0854cf2f 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> > @@ -50,6 +50,12 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
> > if (!dma_resv_trylock(shmem->base.resv))
> > goto unlock_mappings;
> >
> > + /* BO might have become unpurgeable if the last pages_use_count ref
> > + * was dropped, but the BO hasn't been destroyed yet.
> > + */
> > + if (!drm_gem_shmem_is_purgeable(shmem))
> > + goto unlock_mappings;
> > +
> > panfrost_gem_teardown_mappings_locked(bo);
> > drm_gem_shmem_purge_locked(&bo->base);
> > ret = true;
>
