linux-kernel - Re: [PATCH bpf-next v4 2/5] bpf: Add dmabuf iterator

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Message-ID: <CAPhsuW6cTCEwnbfRNX0KDGGs7M+N3xf+EP9FfS5Y_OHyXqs_Qw@mail.gmail.com>
Date: Thu, 8 May 2025 17:27:38 -0700
From: Song Liu <song@...nel.org>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: sumit.semwal@...aro.org, christian.koenig@....com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	skhan@...uxfoundation.org, alexei.starovoitov@...il.com, 
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, android-mm@...gle.com, 
	simona@...ll.ch, eddyz87@...il.com, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	jolsa@...nel.org, mykolal@...com, shuah@...nel.org
Subject: Re: [PATCH bpf-next v4 2/5] bpf: Add dmabuf iterator

On Thu, May 8, 2025 at 11:20 AM T.J. Mercier <tjmercier@...gle.com> 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 <tjmercier@...gle.com>
> Reviewed-by: Christian König <christian.koenig@....com>

Acked-by: Song Liu <song@...nel.org>

With one nitpick below.

> ---
[...]
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 8ff4add71f88..7af2ea839f58 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -634,4 +634,6 @@ int dma_buf_vmap(struct dma_buf *dmabuf, struct iosys_map *map);
>  void dma_buf_vunmap(struct dma_buf *dmabuf, struct iosys_map *map);
>  int dma_buf_vmap_unlocked(struct dma_buf *dmabuf, struct iosys_map *map);
>  void dma_buf_vunmap_unlocked(struct dma_buf *dmabuf, struct iosys_map *map);
> +struct dma_buf *dma_buf_iter_begin(void);
> +struct dma_buf *dma_buf_iter_next(struct dma_buf *dmbuf);
>  #endif /* __DMA_BUF_H__ */
> 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..96b4ba7f0b2c
> --- /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)

nit: It is better to move these two lines later, to where they
are about to be used.

> +
> +static void *dmabuf_iter_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> +       if (*pos)
> +               return NULL;
> +
> +       return dma_buf_iter_begin();
> +}
> +
[...]