Re: [PATCH 1/8] dma-heap: Add proper kref handling on dma-buf heaps

Maxime Ripard
2026-05-05T15:39:17Z

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Hi Boris,

On Tue, May 05, 2026 at 05:20:48PM +0200, Boris Brezillon wrote:
> Hi Ketil,
>
> On Tue, 5 May 2026 16:05:07 +0200
> Ketil Johnsen <ketil.johnsen@arm.com> wrote:
>
> > From: John Stultz <jstultz@google.com>
> >
> > Add proper reference counting on the dma_heap structure. While
> > existing heaps are built-in, we may eventually have heaps loaded
> > from modules, and we'll need to be able to properly handle the
> > references to the heaps
>
> It's weird that this "heap as module" thing is mentioned here, but
> actual robustness to make this safe is not added in the commit or any
> of the following ones.
>
> >
> > Signed-off-by: John Stultz <jstultz@google.com>
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
> > Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> > [Yong: Just add comment for "minor" and "refcount"]
> > Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
> > [Yunfei: Change reviewer's comments]
> > Signed-off-by: Florent Tomasin <florent.tomasin@arm.com>
> > [Florent: Rebase]
> > Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
> > [Ketil: Rebase]
> > ---
> > drivers/dma-buf/dma-heap.c | 29 +++++++++++++++++++++++++++++
> > include/linux/dma-heap.h | 2 ++
> > 2 files changed, 31 insertions(+)
> >
> > diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> > index ac5f8685a6494..9fd365ddbd517 100644
> > --- a/drivers/dma-buf/dma-heap.c
> > +++ b/drivers/dma-buf/dma-heap.c
> > @@ -12,6 +12,7 @@
> > #include <linux/dma-heap.h>
> > #include <linux/err.h>
> > #include <linux/export.h>
> > +#include <linux/kref.h>
> > #include <linux/list.h>
> > #include <linux/nospec.h>
> > #include <linux/syscalls.h>
> > @@ -31,6 +32,7 @@
> > * @heap_devt: heap device node
> > * @list: list head connecting to list of heaps
> > * @heap_cdev: heap char device
> > + * @refcount: reference counter for this heap device
> > *
> > * Represents a heap of memory from which buffers can be made.
> > */
> > @@ -41,6 +43,7 @@ struct dma_heap {
> > dev_t heap_devt;
> > struct list_head list;
> > struct cdev heap_cdev;
> > + struct kref refcount;
> > };
> >
> > static LIST_HEAD(heap_list);
> > @@ -248,6 +251,7 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> > if (!heap)
> > return ERR_PTR(-ENOMEM);
> >
> > + kref_init(&heap->refcount);
> > heap->name = exp_info->name;
> > heap->ops = exp_info->ops;
> > heap->priv = exp_info->priv;
> > @@ -313,6 +317,31 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> > }
> > EXPORT_SYMBOL_NS_GPL(dma_heap_add, "DMA_BUF_HEAP");
> >
> > +static void dma_heap_release(struct kref *ref)
> > +{
> > + struct dma_heap *heap = container_of(ref, struct dma_heap, refcount);
> > + unsigned int minor = MINOR(heap->heap_devt);
> > +
> > + mutex_lock(&heap_list_lock);
> > + list_del(&heap->list);
> > + mutex_unlock(&heap_list_lock);
> > +
> > + device_destroy(dma_heap_class, heap->heap_devt);
> > + cdev_del(&heap->heap_cdev);
> > + xa_erase(&dma_heap_minors, minor);
> > +
> > + kfree(heap);
>
> That's actually problematic, because cdev_del() doesn't guarantee that
> all opened FDs have been closed [1], it just guarantees that no new ones
> can materialize. In order to make that safe, we'd need a
>
> 1. kref_get_unless_zero() in dma_heap_open(), with proper locking around
> the xa_load() to protect against the heap removal that's happening
> here
> 2. a dma_heap_put() in a new dma_heap_close() implementation
> 3. a guarantee that heap implementations won't go away until the last
> ref is dropped, which means ops and all the data needed for this heap
> to satisfy ioctl()s (and more generally every passed at
> dma_heap_add() time) have to stay valid until the last ref is
> dropped. Alternatively, we could restrict this only to in-flight
> ioctl()s, and have the ops replaced by some dummy ops using RCU or a
> rwlock. But I guess live dmabufs allocated on this heap have to
> retain the heap and its implementation anyway.
>
> For record, #3 is already not satisfied by the current tee_heap
> implementation (tee_dma_heap objects can vanish before the dma_heap
> object is gone). The other implementations seem to be fine because they
> are statically linked, and they either have exp_info.priv set to NULL,
> or something that's never released.

That statement won't hold for long, see:
https://lore.kernel.org/r/20260427-dma-buf-heaps-as-modules-v5-0-b6f5678feefc@kernel.org

However, all upstream heaps can be loaded as module, but not unloaded.
So once you get a reference to it, you can assume it will live forever.
That's why we didn't merge that patch before, even though it was discussed:

https://lore.kernel.org/all/CANDhNCqk9Uk4aXHhUsL4hR1GHNmWZnH3C9Np-A02wdi+J3D7tA@mail.gmail.com/

Maxime

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