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 <[email protected]>
>>> ---
>>> 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
>>>
>>
>
