Re: [PATCH v3 7/8] dma-buf: transfer memcg charges via DMA_BUF_IOCTL_XFER_CHARGE

[Christian König]

On 10/28/26 17:09, Albert Esteve wrote:
> On Wed, Oct 28, 2026 at 10:08:19AM +0100, Christian König wrote:
>> On 10/27/26 16:02, Albert Esteve wrote:
>>> dma-heap already supports charging a specific cgroup at allocation
>>> time via charge_fd in DMA_HEAP_IOCTL_ALLOC. However, dma-bufs can
>>> originate from any exporter (DRM, V4L2, etc.), and the allocator may
>>> not know the final owner at allocation time.
>>>
>>> Add DMA_BUF_IOCTL_XFER_CHARGE to transfer a dma-buf's memcg charge
>>> to a different cgroup after allocation. The target is identified by
>>> charge_fd, which may be a cgroupfd or a pidfd.
>>>
>>> The transfer charges the new cgroup before uncharging the old one,
>>> preserving the original charge on failure. An atomic cmpxchg on
>>> dmabuf->memcg prevents races with concurrent transfers.
>>>
>>> Not all dma-buf exporters back their buffers with reclaimable system
>>> memory. Some represent device registers, VRAM, or carved-out pools
>>> where size is not a meaningful memory cost. Add a memcg_account flag
>>> to dma_buf_export_info that exporters set when their buffers should
>>> participate in memcg accounting. The xfer ioctl returns -EOPNOTSUPP
>>> for buffers that do not set the flag. Currently only the system heap
>>> sets it, other exporters can opt in as needed.
>>>
>>> Usage example:
>>>   int cg_fd = open("/sys/fs/cgroup/clients/app1", O_RDONLY | O_DIRECTORY);
>>>
>>>   struct dma_buf_transfer_charge xfer = {
>>>       .charge_fd = cg_fd,
>>>   };
>>>   ioctl(dmabuf_fd, DMA_BUF_IOCTL_XFER_CHARGE, &xfer);
>>>   /* charge now on app1's cgroup */
>>>   close(cg_fd);
>>>
>>> Signed-off-by: Albert Esteve <aesteve@xxxxxxxxxx>
>>> ---
>>>  drivers/dma-buf/dma-buf.c           | 43 +++++++++++++++++++++++++++++++++++++
>>>  drivers/dma-buf/heaps/system_heap.c |  1 +
>>>  include/linux/dma-buf.h             |  5 +++++
>>>  include/linux/memcontrol.h          |  9 ++++++++
>>>  include/uapi/linux/dma-buf.h        | 26 ++++++++++++++++++++++
>>>  mm/memcontrol.c                     | 41 +++++++++++++++++++++++++++++++++++
>>>  6 files changed, 125 insertions(+)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index f31f49ebd0af..07513ecedc10 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -540,6 +540,45 @@ static long dma_buf_import_sync_file(struct dma_buf *dmabuf,
>>>  }
>>>  #endif
>>>  
>>> +/* Transfer a dma-buf's memcg charge; see struct dma_buf_transfer_charge */
>>> +static long dma_buf_xfer_charge(struct dma_buf *dmabuf,
>>> +				const void __user *user_data)
>>> +{
>>> +	struct dma_buf_transfer_charge arg;
>>> +	struct mem_cgroup *memcg;
>>> +	int ret = 0;
>>> +
>>> +	if (copy_from_user(&arg, user_data, sizeof(arg)))
>>> +		return -EFAULT;
>>> +
>>> +	if (!IS_ENABLED(CONFIG_MEMCG))
>>> +		return 0;
>>> +
>>> +	if (arg.flags != 0)
>>> +		return -EINVAL;
>>> +
>>> +	if (!dmabuf->memcg_account)
>>> +		return -EOPNOTSUPP;
>>> +
>>> +	memcg = mem_cgroup_get_from_fd(arg.charge_fd);
>>> +	if (IS_ERR(memcg))
>>> +		return PTR_ERR(memcg);
>>> +
>>> +	if (dmabuf->memcg == memcg)
>>> +		goto skip_transfer;
>>> +
>>> +	if (!mem_cgroup_transfer_dmabuf_charge(dmabuf, memcg)) {
>>> +		ret = -ENOMEM;
>>> +		goto skip_transfer;
>>> +	}
>>> +
>>> +	return 0;
>>> +
>>> +skip_transfer:
>>> +	mem_cgroup_put(memcg);
>>> +	return ret;
>>> +}
>>> +
>>>  static long dma_buf_ioctl(struct file *file,
>>>  			  unsigned int cmd, unsigned long arg)
>>>  {
>>> @@ -590,6 +629,9 @@ static long dma_buf_ioctl(struct file *file,
>>>  		return dma_buf_import_sync_file(dmabuf, (const void __user *)arg);
>>>  #endif
>>>  
>>> +	case DMA_BUF_IOCTL_XFER_CHARGE:
>>> +		return dma_buf_xfer_charge(dmabuf, (const void __user *)arg);
>>> +
>>>  	default:
>>>  		return -ENOTTY;
>>>  	}
>>> @@ -750,6 +792,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
>>>  	dmabuf->size = exp_info->size;
>>>  	dmabuf->exp_name = exp_info->exp_name;
>>>  	dmabuf->owner = exp_info->owner;
>>> +	dmabuf->memcg_account = exp_info->memcg_account;
>>
>> The initial charging should probably be done here as well.
>>
>> This way DMA-buf heaps only needs to pass in the charge_fd and we can re-use the functionality for other DMA-buf exporters as well.
> 
> The problem with doing the charge here is that it happens too late.
> That is the reasoning for this proposal to move all the charging logic
> into heap allocators before pages are allocated.

Oh, that is a really good point we need to talk about.

DMA-buf heaps is actually a rather simple use case, but we have way more complex things.

For example in some use cases the backing store of a DMA-buf is only allocated on first use and only then it needs to be comitted to it's memcg or dmem.

I don't mind implementing it for DMA-buf heaps first, but we should have the uAPI and it's handling so solid that it also works for future use cases.
> What we could do is fill the dmabuf->memcg field when the flag is set
> (i.e., committing into the dmabuf in the patch semantics). But for that
> we'd need adding the memcg we want to charge it to into exp_info in
> order to support charging to cgroups other than the allocator's (for
> the common case would be simpler as we can just look at current).
> Maybe that's what you meant?

At least for the DMA-buf heaps use case that sounds reasonable for now.

Regards,
Christian.

> 
>>
>>>  	spin_lock_init(&dmabuf->name_lock);
>>>  	init_waitqueue_head(&dmabuf->poll);
>>>  	dmabuf->cb_in.poll = dmabuf->cb_out.poll = &dmabuf->poll;
>>> diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
>>> index adeea8cd328b..a764167167ba 100644
>>> --- a/drivers/dma-buf/heaps/system_heap.c
>>> +++ b/drivers/dma-buf/heaps/system_heap.c
>>> @@ -479,6 +479,7 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap,
>>>  	exp_info.size = buffer->len;
>>>  	exp_info.flags = fd_flags;
>>>  	exp_info.priv = buffer;
>>> +	exp_info.memcg_account = true;
>>>  	dmabuf = dma_buf_export(&exp_info);
>>>  	if (IS_ERR(dmabuf)) {
>>>  		ret = PTR_ERR(dmabuf);
>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>>> index 23ec0ff23fba..ac0ec88cab93 100644
>>> --- a/include/linux/dma-buf.h
>>> +++ b/include/linux/dma-buf.h
>>> @@ -433,6 +433,9 @@ struct dma_buf {
>>>  
>>>  	/** @memcg: the cgroup to which this buffer is currently attributed */
>>>  	struct mem_cgroup *memcg;
>>> +
>>> +	/** @memcg_account: account buffer memory to cgroup */
>>> +	bool memcg_account;
>>>  };
>>>  
>>>  /**
>>> @@ -505,6 +508,7 @@ struct dma_buf_attachment {
>>>   * @flags:	mode flags for the file
>>>   * @resv:	reservation-object, NULL to allocate default one
>>>   * @priv:	Attach private data of allocator to this buffer
>>> + * @memcg_account:	account buffer memory to cgroup
>>>   *
>>>   * This structure holds the information required to export the buffer. Used
>>>   * with dma_buf_export() only.
>>> @@ -517,6 +521,7 @@ struct dma_buf_export_info {
>>>  	int flags;
>>>  	struct dma_resv *resv;
>>>  	void *priv;
>>> +	bool memcg_account;
>>>  };
>>>  
>>>  /**
>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>> index 6b30baa79fa9..36fddafbef4e 100644
>>> --- a/include/linux/memcontrol.h
>>> +++ b/include/linux/memcontrol.h
>>> @@ -679,6 +679,9 @@ bool mem_cgroup_reserve_dmabuf(struct mem_cgroup *memcg, gfp_t gfp,
>>>  void mem_cgroup_commit_dmabuf(struct dma_buf *dmabuf, struct mem_cgroup *memcg,
>>>  			      unsigned int nr_pages);
>>>  
>>> +bool mem_cgroup_transfer_dmabuf_charge(struct dma_buf *dmabuf,
>>> +				       struct mem_cgroup *new_memcg);
>>> +
>>>  void __mem_cgroup_uncharge(struct folio *folio);
>>>  
>>>  /**
>>> @@ -1206,6 +1209,12 @@ static inline void mem_cgroup_cancel_dmabuf(struct mem_cgroup *memcg,
>>>  {
>>>  }
>>>  
>>> +static inline bool mem_cgroup_transfer_dmabuf_charge(struct dma_buf *dmabuf,
>>> +						     struct mem_cgroup *new_memcg)
>>> +{
>>> +	return true;
>>> +}
>>> +
>>>  static inline void mem_cgroup_uncharge(struct folio *folio)
>>>  {
>>>  }
>>> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
>>> index e827c9d20c5d..ea957f4b1d38 100644
>>> --- a/include/uapi/linux/dma-buf.h
>>> +++ b/include/uapi/linux/dma-buf.h
>>> @@ -168,6 +168,31 @@ struct dma_buf_import_sync_file {
>>>  	__s32 fd;
>>>  };
>>>  
>>> +/**
>>> + * struct dma_buf_transfer_charge - Transfer a dma-buf's memcg charge
>>> + *
>>> + * Any holder of a dma-buf file descriptor can perform a
>>> + * DMA_BUF_IOCTL_XFER_CHARGE to attribute the buffer's memory
>>> + * cost to a specific cgroup, or move it from one cgroup to
>>> + * another.  The ioctl charges the new cgroup before uncharging
>>> + * the old one atomically.  On failure the original charge is
>>> + * preserved.  If the buffer has no existing charge, a new
>>> + * charge is established.
>>> + */
>>> +struct dma_buf_transfer_charge {
>>> +	/** @flags: Reserved, must be zero */
>>> +	__u32 flags; /* unused for now */
>>> +	/**
>>> +	 * @charge_fd: File descriptor identifying the target cgroup.
>>> +	 *
>>> +	 * May be either a pidfd, in which case the charge moves to the
>>> +	 * memory cgroup of the referenced process, or a cgroupfd obtained
>>> +	 * by opening a cgroup v2 directory, in which case the charge moves
>>> +	 * directly to that cgroup's memory controller.
>>> +	 */
>>> +	__u32 charge_fd;
>>> +};
>>> +
>>>  #define DMA_BUF_BASE		'b'
>>>  #define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
>>>  
>>> @@ -179,5 +204,6 @@ struct dma_buf_import_sync_file {
>>>  #define DMA_BUF_SET_NAME_B	_IOW(DMA_BUF_BASE, 1, __u64)
>>>  #define DMA_BUF_IOCTL_EXPORT_SYNC_FILE	_IOWR(DMA_BUF_BASE, 2, struct dma_buf_export_sync_file)
>>>  #define DMA_BUF_IOCTL_IMPORT_SYNC_FILE	_IOW(DMA_BUF_BASE, 3, struct dma_buf_import_sync_file)
>>> +#define DMA_BUF_IOCTL_XFER_CHARGE	_IOW(DMA_BUF_BASE, 4, struct dma_buf_transfer_charge)
>>>  
>>>  #endif
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index 7b841b905759..3adc84fe6668 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -5786,6 +5786,47 @@ void mem_cgroup_cancel_dmabuf(struct mem_cgroup *memcg, unsigned int nr_pages)
>>>  		refill_stock(memcg, nr_pages);
>>>  }
>>>  
>>> +/**
>>> + * mem_cgroup_transfer_dmabuf_charge - move a dma-buf charge
>>> + * @dmabuf: buffer to transfer charge
>>> + * @new_memcg: target memcg; caller must hold a reference
>>> + *
>>> + * Transfers the charge for @dmabuf to @new_memcg and, on
>>> + * success, stores @new_memcg in @dmabuf->memcg.
>>> + * Returns %true on success, %false if the charge against @new_memcg
>>> + * failed or a concurrent transfer raced on @dmabuf->memcg.
>>> + * On failure the caller retains its reference to @new_memcg.
>>> + */
>>> +bool mem_cgroup_transfer_dmabuf_charge(struct dma_buf *dmabuf,
>>> +				       struct mem_cgroup *new_memcg)
>>
>> I would make that more generic instead of dmabuf specific. E.g. something like this:
>>
>> bool mem_cgroup_transfer_charge(struct struct mem_cgroup **ptr_memcg, struct mem_cgroup *new_memcg, enum memcg_stat_item item, unsigned int nr_pages)
> 
> Ah yes, I like this generalization. And the implementation remains
> mostly the same. Thanks!
> 
>>
>>> +{
>>> +	struct mem_cgroup *old_memcg = READ_ONCE(dmabuf->memcg);
>>
>>> +	unsigned int nr_pages = PAGE_ALIGN(dmabuf->size) >> PAGE_SHIFT;
>>
>> This way we can also document in the DMA-buf caller that this conversation must be ok when memcg_account is true.
>>
>> Regards,
>> Christian.
>>
>>> +
>>> +	if (mem_cgroup_disabled())
>>> +		return true;
>>> +
>>> +	if (!mem_cgroup_reserve_dmabuf(new_memcg, GFP_KERNEL, nr_pages))
>>> +		return false;
>>> +
>>> +	mod_memcg_state(new_memcg, MEMCG_DMABUF, nr_pages);
>>> +
>>> +	if (cmpxchg(&dmabuf->memcg, old_memcg, new_memcg) != old_memcg) {
>>> +		mod_memcg_state(new_memcg, MEMCG_DMABUF, -nr_pages);
>>> +		mem_cgroup_cancel_dmabuf(new_memcg, nr_pages);
>>> +		return false;
>>> +	}
>>> +
>>> +	if (old_memcg) {
>>> +		/* Uncharge old cgroup */
>>> +		mod_memcg_state(old_memcg, MEMCG_DMABUF, -nr_pages);
>>> +		mem_cgroup_cancel_dmabuf(old_memcg, nr_pages);
>>> +		mem_cgroup_put(old_memcg);
>>> +	}
>>> +
>>> +	return true;
>>> +}
>>> +
>>>  /**
>>>   * mem_cgroup_uncharge_dmabuf - uncharge a dma-buf from its memcg
>>>   * @dmabuf: buffer to uncharge
>>>
>>
>