On Mon, May 5, 2025 at 9:56 AM Christian König <christian.koe...@amd.com> wrote:
>
> On 5/5/25 18:33, T.J. Mercier wrote:
> > On Mon, May 5, 2025 at 4:17 AM Christian König <christian.koe...@amd.com>
> > wrote:
> >>
> >> On 5/5/25 00:41, 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>
> >>> ---
> >>> kernel/bpf/Makefile | 3 +
> >>> kernel/bpf/dmabuf_iter.c | 134 +++++++++++++++++++++++++++++++++++++++
> >>> 2 files changed, 137 insertions(+)
> >>> create mode 100644 kernel/bpf/dmabuf_iter.c
> >>>
> >>> 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..968762e11f73
> >>> --- /dev/null
> >>> +++ b/kernel/bpf/dmabuf_iter.c
> >>> @@ -0,0 +1,134 @@
> >>> +// 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 struct dma_buf *get_next_dmabuf(struct dma_buf *dmabuf)
> >>> +{
> >>> + struct dma_buf *ret = NULL;
> >>> +
> >>> + /*
> >>> + * Look for the first/next buffer we can obtain a reference to.
> >>> + *
> >>> + * 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 of this program may not already own a reference to the
> >>> buffer.
> >>> + */
> >>> + mutex_lock(&dmabuf_list_mutex);
> >>> + if (dmabuf) {
> >>
> >> That looks like you try to mangle the start and next functionality in just
> >> one function.
> >>
> >> I would just inline that into the dmabuf_iter_seq_start() and
> >> dmabuf_iter_seq_next() functions.
> >
> > Primarily this is to share between the open coded iterator (next
> > patch) and this normal iterator since I didn't want to duplicate the
> > same list traversal code across both of them.
>
> Ah, ok that makes a bit more sense. It would still be nicer if it's in two
> functions since the logic doesn't share anything common except for taking the
> lock as far as I can seee.
>
> >>
> >>
> >>> + dma_buf_put(dmabuf);
> >>> + list_for_each_entry_continue(dmabuf, &dmabuf_list,
> >>> list_node) {
> >>
> >> That you can put the DMA-buf and then still uses it in
> >> list_for_each_entry_continue() only works because the mutex is locked in
> >> the destroy path.
> >
> > Yup, this was deliberate.
> >>
> >>
> >> I strongly suggest to just put those two functions into
> >> drivers/dma-buf/dma-buf.c right next to the __dma_buf_debugfs_list_add()
> >> and __dma_buf_debugfs_list_del() functions.
> >
> > By two functions, you mean a get_first_dmabuf(void) and a
> > get_next_dmabuf(struct dma_buf*)? To make the dma_buf_put() call a
> > little less scary since all the mutex ops are right there?
>
> Yes, exactly that's the idea. The comment above is good to have as well, but
> it only works one way.
>
> If somebody changes the DMA-buf code without looking at this here we are
> busted.
Sounds good, will do. Thanks.
>
> Regards,
> Christian.
>
> >>
> >>
> >> Apart from those style suggestions looks good to me from the technical
> >> side, but I'm not an expert for the BPF stuff.
> >>
> >> Regards,
> >> Christian.
> >
> > Thanks for your comments and reviews!
> >
> >>> + if (file_ref_get(&dmabuf->file->f_ref)) {
> >>> + ret = dmabuf;
> >>> + break;
> >>> + }
> >>> + }
> >>> + } else {
> >>> + 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;
> >>> +}
> >>> +
> >>> +static void *dmabuf_iter_seq_start(struct seq_file *seq, loff_t *pos)
> >>> +{
> >>> + if (*pos)
> >>> + return NULL;
> >>> +
> >>> + return get_next_dmabuf(NULL);
> >>> +}
> >>> +
> >>> +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);
> >>
>
