On 5/7/25 19:36, T.J. Mercier wrote:
> On Wed, May 7, 2025 at 1:15 AM Christian König <christian.koe...@amd.com>
> wrote:
>>
>> On 5/7/25 02:10, T.J. Mercier wrote:
>>> The dmabuf iterator traverses the list of all DMA buffers.
>>>
>>> DMA buffers are refcounted through their associated struct file. A
>>> reference is taken on each buffer as the list is iterated to ensure each
>>> buffer persists for the duration of the bpf program execution without
>>> holding the list mutex.
>>>
>>> Signed-off-by: T.J. Mercier <tjmerc...@google.com>
>>> ---
>>> drivers/dma-buf/dma-buf.c | 64 ++++++++++++++++++++++++
>>> include/linux/dma-buf.h | 3 ++
>>> kernel/bpf/Makefile | 3 ++
>>> kernel/bpf/dmabuf_iter.c | 102 ++++++++++++++++++++++++++++++++++++++
>>> 4 files changed, 172 insertions(+)
>>> create mode 100644 kernel/bpf/dmabuf_iter.c
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index 8d151784e302..9fee2788924e 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -19,7 +19,9 @@
>>> #include <linux/anon_inodes.h>
>>> #include <linux/export.h>
>>> #include <linux/debugfs.h>
>>> +#include <linux/list.h>
>>> #include <linux/module.h>
>>> +#include <linux/mutex.h>
>>> #include <linux/seq_file.h>
>>> #include <linux/sync_file.h>
>>> #include <linux/poll.h>
>>> @@ -55,6 +57,68 @@ static void __dma_buf_list_del(struct dma_buf *dmabuf)
>>> mutex_unlock(&dmabuf_list_mutex);
>>> }
>>>
>>> +/**
>>> + * get_first_dmabuf - begin iteration through global list of DMA-bufs
>>
>> As far as I can see that looks really good.
>>
>> The only thing I'm questioning a little bit is that the name
>> get_first_dmabuf() just doesn't sound so well to me.
>>
>> I'm a fan of keeping the object you work with first in the naming and it
>> should somehow express that this iters over the global list of all buffers.
>> Maybe something like dmabuf_get_first_globally or dmabuf_get_first_instance.
>>
>> Feel free to add my rb if any of those suggestions are used, but I'm
>> completely open other ideas as well.
>>
>> Regards,
>> Christian.
>>
> Yeah you're right. "first" is actually a little misleading too, since
> the most recently exported buffer will be at the list head, not the
> oldest buffer. But buffer age or ordering doesn't really matter here
> as long as we get through all of them.
>
> So I'm thinking dma_buf_iter_begin() and dma_buf_iter_next() would be
> better names. Similar to seq_start / seq_next or bpf's iter_<type>_new
> / iter_<type>_next.
Yeah, dma_buf_iter_begin/next works for me as well.
Feel free to add my rb when you use those names.
Regards,
Christian.
>
>>> + *
>>> + * Returns the first buffer in the global list of DMA-bufs that's not in
>>> the
>>> + * process of being destroyed. Increments that buffer's reference count to
>>> + * prevent buffer destruction. Callers must release the reference, either
>>> by
>>> + * continuing iteration with get_next_dmabuf(), or with dma_buf_put().
>>> + *
>>> + * Returns NULL If no active buffers are present.
>>> + */
>>> +struct dma_buf *get_first_dmabuf(void)
>>> +{
>>> + struct dma_buf *ret = NULL, *dmabuf;
>>> +
>>> + /*
>>> + * The list mutex does not protect a dmabuf's refcount, so it can be
>>> + * zeroed while we are iterating. We cannot call get_dma_buf() since
>>> the
>>> + * caller may not already own a reference to the buffer.
>>> + */
>>> + mutex_lock(&dmabuf_list_mutex);
>>> + list_for_each_entry(dmabuf, &dmabuf_list, list_node) {
>>> + if (file_ref_get(&dmabuf->file->f_ref)) {
>>> + ret = dmabuf;
>>> + break;
>>> + }
>>> + }
>>> + mutex_unlock(&dmabuf_list_mutex);
>>> + return ret;
>>> +}
>>> +
>>> +/**
>>> + * get_next_dmabuf - continue iteration through global list of DMA-bufs
>>> + * @dmabuf: [in] pointer to dma_buf
>>> + *
>>> + * Decrements the reference count on the provided buffer. Returns the next
>>> + * buffer from the remainder of the global list of DMA-bufs with its
>>> reference
>>> + * count incremented. Callers must release the reference, either by
>>> continuing
>>> + * iteration with get_next_dmabuf(), or with dma_buf_put().
>>> + *
>>> + * Returns NULL If no additional active buffers are present.
>>> + */
>>> +struct dma_buf *get_next_dmabuf(struct dma_buf *dmabuf)
>>> +{
>>> + struct dma_buf *ret = NULL;
>>> +
>>> + /*
>>> + * The list mutex does not protect a dmabuf's refcount, so it can be
>>> + * zeroed while we are iterating. We cannot call get_dma_buf() since
>>> the
>>> + * caller may not already own a reference to the buffer.
>>> + */
>>> + mutex_lock(&dmabuf_list_mutex);
>>> + dma_buf_put(dmabuf);
>>> + list_for_each_entry_continue(dmabuf, &dmabuf_list, list_node) {
>>> + if (file_ref_get(&dmabuf->file->f_ref)) {
>>> + ret = dmabuf;
>>> + break;
>>> + }
>>> + }
>>> + mutex_unlock(&dmabuf_list_mutex);
>>> + return ret;
>>> +}
>>> +
>>> static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int
>>> buflen)
>>> {
>>> struct dma_buf *dmabuf;
>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>>> index 8ff4add71f88..1820f6db6e58 100644
>>> --- a/include/linux/dma-buf.h
>>> +++ b/include/linux/dma-buf.h
>>> @@ -568,6 +568,9 @@ static inline void get_dma_buf(struct dma_buf *dmabuf)
>>> get_file(dmabuf->file);
>>> }
>>>
>>> +struct dma_buf *get_first_dmabuf(void);
>>> +struct dma_buf *get_next_dmabuf(struct dma_buf *dmbuf);
>>> +
>>> /**
>>> * dma_buf_is_dynamic - check if a DMA-buf uses dynamic mappings.
>>> * @dmabuf: the DMA-buf to check
>>> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
>>> index 70502f038b92..3a335c50e6e3 100644
>>> --- a/kernel/bpf/Makefile
>>> +++ b/kernel/bpf/Makefile
>>> @@ -53,6 +53,9 @@ obj-$(CONFIG_BPF_SYSCALL) += relo_core.o
>>> obj-$(CONFIG_BPF_SYSCALL) += btf_iter.o
>>> obj-$(CONFIG_BPF_SYSCALL) += btf_relocate.o
>>> obj-$(CONFIG_BPF_SYSCALL) += kmem_cache_iter.o
>>> +ifeq ($(CONFIG_DMA_SHARED_BUFFER),y)
>>> +obj-$(CONFIG_BPF_SYSCALL) += dmabuf_iter.o
>>> +endif
>>>
>>> CFLAGS_REMOVE_percpu_freelist.o = $(CC_FLAGS_FTRACE)
>>> CFLAGS_REMOVE_bpf_lru_list.o = $(CC_FLAGS_FTRACE)
>>> diff --git a/kernel/bpf/dmabuf_iter.c b/kernel/bpf/dmabuf_iter.c
>>> new file mode 100644
>>> index 000000000000..80bca8239c6d
>>> --- /dev/null
>>> +++ b/kernel/bpf/dmabuf_iter.c
>>> @@ -0,0 +1,102 @@
>>> +// SPDX-License-Identifier: GPL-2.0-only
>>> +/* Copyright (c) 2025 Google LLC */
>>> +#include <linux/bpf.h>
>>> +#include <linux/btf_ids.h>
>>> +#include <linux/dma-buf.h>
>>> +#include <linux/kernel.h>
>>> +#include <linux/seq_file.h>
>>> +
>>> +BTF_ID_LIST_SINGLE(bpf_dmabuf_btf_id, struct, dma_buf)
>>> +DEFINE_BPF_ITER_FUNC(dmabuf, struct bpf_iter_meta *meta, struct dma_buf
>>> *dmabuf)
>>> +
>>> +static void *dmabuf_iter_seq_start(struct seq_file *seq, loff_t *pos)
>>> +{
>>> + if (*pos)
>>> + return NULL;
>>> +
>>> + return get_first_dmabuf();
>>> +}
>>> +
>>> +static void *dmabuf_iter_seq_next(struct seq_file *seq, void *v, loff_t
>>> *pos)
>>> +{
>>> + struct dma_buf *dmabuf = v;
>>> +
>>> + ++*pos;
>>> +
>>> + return get_next_dmabuf(dmabuf);
>>> +}
>>> +
>>> +struct bpf_iter__dmabuf {
>>> + __bpf_md_ptr(struct bpf_iter_meta *, meta);
>>> + __bpf_md_ptr(struct dma_buf *, dmabuf);
>>> +};
>>> +
>>> +static int __dmabuf_seq_show(struct seq_file *seq, void *v, bool in_stop)
>>> +{
>>> + struct bpf_iter_meta meta = {
>>> + .seq = seq,
>>> + };
>>> + struct bpf_iter__dmabuf ctx = {
>>> + .meta = &meta,
>>> + .dmabuf = v,
>>> + };
>>> + struct bpf_prog *prog = bpf_iter_get_info(&meta, in_stop);
>>> +
>>> + if (prog)
>>> + return bpf_iter_run_prog(prog, &ctx);
>>> +
>>> + return 0;
>>> +}
>>> +
>>> +static int dmabuf_iter_seq_show(struct seq_file *seq, void *v)
>>> +{
>>> + return __dmabuf_seq_show(seq, v, false);
>>> +}
>>> +
>>> +static void dmabuf_iter_seq_stop(struct seq_file *seq, void *v)
>>> +{
>>> + struct dma_buf *dmabuf = v;
>>> +
>>> + if (dmabuf)
>>> + dma_buf_put(dmabuf);
>>> +}
>>> +
>>> +static const struct seq_operations dmabuf_iter_seq_ops = {
>>> + .start = dmabuf_iter_seq_start,
>>> + .next = dmabuf_iter_seq_next,
>>> + .stop = dmabuf_iter_seq_stop,
>>> + .show = dmabuf_iter_seq_show,
>>> +};
>>> +
>>> +static void bpf_iter_dmabuf_show_fdinfo(const struct bpf_iter_aux_info
>>> *aux,
>>> + struct seq_file *seq)
>>> +{
>>> + seq_puts(seq, "dmabuf iter\n");
>>> +}
>>> +
>>> +static const struct bpf_iter_seq_info dmabuf_iter_seq_info = {
>>> + .seq_ops = &dmabuf_iter_seq_ops,
>>> + .init_seq_private = NULL,
>>> + .fini_seq_private = NULL,
>>> + .seq_priv_size = 0,
>>> +};
>>> +
>>> +static struct bpf_iter_reg bpf_dmabuf_reg_info = {
>>> + .target = "dmabuf",
>>> + .feature = BPF_ITER_RESCHED,
>>> + .show_fdinfo = bpf_iter_dmabuf_show_fdinfo,
>>> + .ctx_arg_info_size = 1,
>>> + .ctx_arg_info = {
>>> + { offsetof(struct bpf_iter__dmabuf, dmabuf),
>>> + PTR_TO_BTF_ID_OR_NULL },
>>> + },
>>> + .seq_info = &dmabuf_iter_seq_info,
>>> +};
>>> +
>>> +static int __init dmabuf_iter_init(void)
>>> +{
>>> + bpf_dmabuf_reg_info.ctx_arg_info[0].btf_id = bpf_dmabuf_btf_id[0];
>>> + return bpf_iter_reg_target(&bpf_dmabuf_reg_info);
>>> +}
>>> +
>>> +late_initcall(dmabuf_iter_init);
>>
