[Linaro-mm-sig] Re: [PATCH v4 3/8] dma-buf: transfer memcg charges via DMA_BUF_IOCTL_XFER_CHARGE

On 11/4/26 12:09, 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. Concurrent transfers are serialized by the dma-buf reservation lock, so lockdep covers the locking order of the charge path early. The transfer implementation is generic (mem_cgroup_transfer_charge) and not dma-buf specific, so it can be reused by other subsystems.

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. The xfer ioctl returns -EOPNOTSUPP for buffers whose memcg is not set. Exporters opt in by passing a memcg through dma_buf_export_info at export time. 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@redhat.com

drivers/dma-buf/dma-buf.c | 42 ++++++++++++++++++++++++++++++++++++++++++ include/linux/dma-buf.h | 3 ++- include/linux/memcontrol.h | 13 +++++++++++++ include/uapi/linux/dma-buf.h | 26 ++++++++++++++++++++++++++ mm/memcontrol.c | 42 ++++++++++++++++++++++++++++++++++++++++++ 5 files changed, 125 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 837666273550..3bd26fbb1109 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;
• unsigned int nr_pages = PAGE_ALIGN(dmabuf->size) >> PAGE_SHIFT;
• 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;
  

• memcg = mem_cgroup_get_from_fd(arg.charge_fd);
• if (IS_ERR(memcg))

• return PTR_ERR(memcg);
  

• dma_resv_lock(dmabuf->resv, NULL);
• if (!dmabuf->memcg) {

• ret = -EOPNOTSUPP;
  

• goto out;
  

• }
• if (dmabuf->memcg != memcg &&

•    !mem_cgroup_transfer_charge(&dmabuf->memcg, memcg, MEMCG_DMABUF, nr_pages))
  

Please move the calculation of nr_pages after taking the dma_resv_lock().

We currently don't have anybody dynamically changing DMA-buf sizes but just to be on the save side.

With that done Reviewed-by: Christian König christian.koenig@amd.com

Regards, Christian.

• ret = -ENOMEM;
  

+out:

• dma_resv_unlock(dmabuf->resv);
• 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; }

diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h index 85dd747c9b36..b01149d26b44 100644 --- a/include/linux/dma-buf.h +++ b/include/linux/dma-buf.h @@ -507,7 +507,8 @@ struct dma_buf_attachment {

• @priv: Attach private data of allocator to this buffer
• @memcg: memcg to charge for this buffer's memory. Set only for

• buffers backed by system memory where @size reflects the
  

• • actual allocation in bytes.
    

• • actual allocation in bytes. Enables charge transfer via
    

• • DMA_BUF_IOCTL_XFER_CHARGE.
    

  • This structure holds the information required to export the buffer. Used
  • with dma_buf_export() only.

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index f8ec751b7d7b..af02d9777fe9 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -676,6 +676,11 @@ struct mem_cgroup *mem_cgroup_get_from_fd(int fd); bool mem_cgroup_reserve_dmabuf(struct mem_cgroup *memcg, gfp_t gfp, unsigned int nr_pages); +bool mem_cgroup_transfer_charge(struct mem_cgroup **ptr_memcg,

• 		struct mem_cgroup *new_memcg,
  

• 		enum memcg_stat_item item,
  

• 		unsigned int nr_pages);
  

void __mem_cgroup_uncharge(struct folio *folio); /** @@ -1197,6 +1202,14 @@ static inline void mem_cgroup_cancel_dmabuf(struct mem_cgroup *memcg, { } +static inline bool mem_cgroup_transfer_charge(struct mem_cgroup **ptr_memcg,

• 			      struct mem_cgroup *new_memcg,
  

• 			      enum memcg_stat_item item,
  

• 			      unsigned int nr_pages)
  

+{

• 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..0aaf2e7d593b 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 memcg charge, the
• • ioctl returns -EOPNOTSUPP.
• */

+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 17fdefc56b97..3e717b5663c8 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5752,6 +5752,48 @@ void mem_cgroup_cancel_dmabuf(struct mem_cgroup *memcg, unsigned int nr_pages) } EXPORT_SYMBOL_GPL(mem_cgroup_cancel_dmabuf); +/**

• • mem_cgroup_transfer_charge - move a charge between memcgs
• • @ptr_memcg: pointer to the memcg to transfer from
• • @new_memcg: target memcg; caller must hold a reference
• • @item: stat counter to adjust
• • @nr_pages: number of pages to transfer
• • Transfers @nr_pages of charge from *@ptr_memcg to @new_memcg and, on
• • success, stores @new_memcg in *@ptr_memcg.
• • Updates to *@ptr_memcg must be serialized by the caller.
• • Returns %true on success, %false if the charge against @new_memcg
• • failed.
• */

+bool mem_cgroup_transfer_charge(struct mem_cgroup **ptr_memcg,

• 		struct mem_cgroup *new_memcg,
  

• 		enum memcg_stat_item item,
  

• 		unsigned int nr_pages)
  

+{

• struct mem_cgroup *old_memcg = *ptr_memcg;
• if (!new_memcg || mem_cgroup_disabled())

• return true;
  

• if (try_charge(new_memcg, GFP_KERNEL, nr_pages))

• return false;
  

• mod_memcg_state(new_memcg, item, nr_pages);
• css_get(&new_memcg->css);
• *ptr_memcg = new_memcg;
• if (old_memcg) {

• /* Uncharge old cgroup */
  

• mod_memcg_state(old_memcg, item, -nr_pages);
  

• if (!mem_cgroup_is_root(old_memcg))
  

• 	refill_stock(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