Message-ID: <CAO_48GENCZrXNCgXnW4rujNMHk8d9QrXoACT1zpJhWR0r0jACw@mail.gmail.com>
Date: Mon, 19 Jan 2026 18:38:27 +0530
From: Sumit Semwal <sumit.semwal@...aro.org>
To: Christian König <christian.koenig@....com>
Cc: "T.J. Mercier" <tjmercier@...gle.com>, Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] dma-buf: Remove DMA-BUF sysfs stats

Hi T J,

On Mon, 19 Jan 2026 at 15:15, Christian König <christian.koenig@....com> wrote:
>
> On 1/16/26 20:05, T.J. Mercier wrote:
> > Commit bdb8d06dfefd ("dmabuf: Add the capability to expose DMA-BUF stats
> > in sysfs") added dmabuf statistics to sysfs in 2021 under
> > CONFIG_DMABUF_SYSFS_STATS. After being used in production, performance
> > problems were discovered leading to its deprecation in 2022 in commit
> > e0a9f1fe206a ("dma-buf: deprecate DMABUF_SYSFS_STATS"). Some of the
> > problems with this interface were discussed in my LPC 2025 talk. [1][2]
> >
> > Android was probably the last user of the interface, which has since
> > been migrated to use the dmabuf BPF iterator [3] to obtain the same
> > information more cheaply. As promised in that series, now that the
> > longterm stable 6.18 kernel has been released let's remove the sysfs
> > dmabuf statistics from the kernel.
> >
> > [1] https://www.youtube.com/watch?v=D83qygudq9c
> > [2] https://lpc.events/event/19/contributions/2118/
> > [3] https://lore.kernel.org/all/20250522230429.941193-1-tjmercier@google.com/
> >
> > Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
>
> Oh yes please :)
>
> Reviewed-by: Christian König <christian.koenig@....com>
Thank you for your patch, LGTM :)

If its not pushed yet, I'll push it tonight.

Best,
Sumit
>
> > ---
> >  .../ABI/testing/sysfs-kernel-dmabuf-buffers   |  24 ---
> >  Documentation/driver-api/dma-buf.rst          |   5 -
> >  drivers/dma-buf/Kconfig                       |  15 --
> >  drivers/dma-buf/Makefile                      |   1 -
> >  drivers/dma-buf/dma-buf-sysfs-stats.c         | 202 ------------------
> >  drivers/dma-buf/dma-buf-sysfs-stats.h         |  35 ---
> >  drivers/dma-buf/dma-buf.c                     |  18 --
> >  include/linux/dma-buf.h                       |  12 --
> >  8 files changed, 312 deletions(-)
> >  delete mode 100644 Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
> >  delete mode 100644 drivers/dma-buf/dma-buf-sysfs-stats.c
> >  delete mode 100644 drivers/dma-buf/dma-buf-sysfs-stats.h
> >
> > diff --git a/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers b/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
> > deleted file mode 100644
> > index 5d3bc997dc64..000000000000
> > --- a/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
> > +++ /dev/null
> > @@ -1,24 +0,0 @@
> > -What:                /sys/kernel/dmabuf/buffers
> > -Date:                May 2021
> > -KernelVersion:       v5.13
> > -Contact:     Hridya Valsaraju <hridya@...gle.com>
> > -Description: The /sys/kernel/dmabuf/buffers directory contains a
> > -             snapshot of the internal state of every DMA-BUF.
> > -             /sys/kernel/dmabuf/buffers/<inode_number> will contain the
> > -             statistics for the DMA-BUF with the unique inode number
> > -             <inode_number>
> > -Users:               kernel memory tuning/debugging tools
> > -
> > -What:                /sys/kernel/dmabuf/buffers/<inode_number>/exporter_name
> > -Date:                May 2021
> > -KernelVersion:       v5.13
> > -Contact:     Hridya Valsaraju <hridya@...gle.com>
> > -Description: This file is read-only and contains the name of the exporter of
> > -             the DMA-BUF.
> > -
> > -What:                /sys/kernel/dmabuf/buffers/<inode_number>/size
> > -Date:                May 2021
> > -KernelVersion:       v5.13
> > -Contact:     Hridya Valsaraju <hridya@...gle.com>
> > -Description: This file is read-only and specifies the size of the DMA-BUF in
> > -             bytes.
> > diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
> > index 29abf1eebf9f..2f36c21d9948 100644
> > --- a/Documentation/driver-api/dma-buf.rst
> > +++ b/Documentation/driver-api/dma-buf.rst
> > @@ -125,11 +125,6 @@ Implicit Fence Poll Support
> >  .. kernel-doc:: drivers/dma-buf/dma-buf.c
> >     :doc: implicit fence polling
> >
> > -DMA-BUF statistics
> > -~~~~~~~~~~~~~~~~~~
> > -.. kernel-doc:: drivers/dma-buf/dma-buf-sysfs-stats.c
> > -   :doc: overview
> > -
> >  DMA Buffer ioctls
> >  ~~~~~~~~~~~~~~~~~
> >
> > diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> > index fdd823e446cc..012d22e941d6 100644
> > --- a/drivers/dma-buf/Kconfig
> > +++ b/drivers/dma-buf/Kconfig
> > @@ -75,21 +75,6 @@ menuconfig DMABUF_HEAPS
> >         allows userspace to allocate dma-bufs that can be shared
> >         between drivers.
> >
> > -menuconfig DMABUF_SYSFS_STATS
> > -     bool "DMA-BUF sysfs statistics (DEPRECATED)"
> > -     depends on DMA_SHARED_BUFFER
> > -     help
> > -        Choose this option to enable DMA-BUF sysfs statistics
> > -        in location /sys/kernel/dmabuf/buffers.
> > -
> > -        /sys/kernel/dmabuf/buffers/<inode_number> will contain
> > -        statistics for the DMA-BUF with the unique inode number
> > -        <inode_number>.
> > -
> > -        This option is deprecated and should sooner or later be removed.
> > -        Android is the only user of this and it turned out that this resulted
> > -        in quite some performance problems.
> > -
> >  source "drivers/dma-buf/heaps/Kconfig"
> >
> >  endmenu
> > diff --git a/drivers/dma-buf/Makefile b/drivers/dma-buf/Makefile
> > index 2008fb7481b3..7a85565d906b 100644
> > --- a/drivers/dma-buf/Makefile
> > +++ b/drivers/dma-buf/Makefile
> > @@ -6,7 +6,6 @@ obj-$(CONFIG_DMABUF_HEAPS)    += heaps/
> >  obj-$(CONFIG_SYNC_FILE)              += sync_file.o
> >  obj-$(CONFIG_SW_SYNC)                += sw_sync.o sync_debug.o
> >  obj-$(CONFIG_UDMABUF)                += udmabuf.o
> > -obj-$(CONFIG_DMABUF_SYSFS_STATS) += dma-buf-sysfs-stats.o
> >
> >  dmabuf_selftests-y := \
> >       selftest.o \
> > diff --git a/drivers/dma-buf/dma-buf-sysfs-stats.c b/drivers/dma-buf/dma-buf-sysfs-stats.c
> > deleted file mode 100644
> > index b5b62e40ccc1..000000000000
> > --- a/drivers/dma-buf/dma-buf-sysfs-stats.c
> > +++ /dev/null
> > @@ -1,202 +0,0 @@
> > -// SPDX-License-Identifier: GPL-2.0-only
> > -/*
> > - * DMA-BUF sysfs statistics.
> > - *
> > - * Copyright (C) 2021 Google LLC.
> > - */
> > -
> > -#include <linux/dma-buf.h>
> > -#include <linux/dma-resv.h>
> > -#include <linux/kobject.h>
> > -#include <linux/printk.h>
> > -#include <linux/slab.h>
> > -#include <linux/sysfs.h>
> > -
> > -#include "dma-buf-sysfs-stats.h"
> > -
> > -#define to_dma_buf_entry_from_kobj(x) container_of(x, struct dma_buf_sysfs_entry, kobj)
> > -
> > -/**
> > - * DOC: overview
> > - *
> > - * ``/sys/kernel/debug/dma_buf/bufinfo`` provides an overview of every DMA-BUF
> > - * in the system. However, since debugfs is not safe to be mounted in
> > - * production, procfs and sysfs can be used to gather DMA-BUF statistics on
> > - * production systems.
> > - *
> > - * The ``/proc/<pid>/fdinfo/<fd>`` files in procfs can be used to gather
> > - * information about DMA-BUF fds. Detailed documentation about the interface
> > - * is present in Documentation/filesystems/proc.rst.
> > - *
> > - * Unfortunately, the existing procfs interfaces can only provide information
> > - * about the DMA-BUFs for which processes hold fds or have the buffers mmapped
> > - * into their address space. This necessitated the creation of the DMA-BUF sysfs
> > - * statistics interface to provide per-buffer information on production systems.
> > - *
> > - * The interface at ``/sys/kernel/dmabuf/buffers`` exposes information about
> > - * every DMA-BUF when ``CONFIG_DMABUF_SYSFS_STATS`` is enabled.
> > - *
> > - * The following stats are exposed by the interface:
> > - *
> > - * * ``/sys/kernel/dmabuf/buffers/<inode_number>/exporter_name``
> > - * * ``/sys/kernel/dmabuf/buffers/<inode_number>/size``
> > - *
> > - * The information in the interface can also be used to derive per-exporter
> > - * statistics. The data from the interface can be gathered on error conditions
> > - * or other important events to provide a snapshot of DMA-BUF usage.
> > - * It can also be collected periodically by telemetry to monitor various metrics.
> > - *
> > - * Detailed documentation about the interface is present in
> > - * Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers.
> > - */
> > -
> > -struct dma_buf_stats_attribute {
> > -     struct attribute attr;
> > -     ssize_t (*show)(struct dma_buf *dmabuf,
> > -                     struct dma_buf_stats_attribute *attr, char *buf);
> > -};
> > -#define to_dma_buf_stats_attr(x) container_of(x, struct dma_buf_stats_attribute, attr)
> > -
> > -static ssize_t dma_buf_stats_attribute_show(struct kobject *kobj,
> > -                                         struct attribute *attr,
> > -                                         char *buf)
> > -{
> > -     struct dma_buf_stats_attribute *attribute;
> > -     struct dma_buf_sysfs_entry *sysfs_entry;
> > -     struct dma_buf *dmabuf;
> > -
> > -     attribute = to_dma_buf_stats_attr(attr);
> > -     sysfs_entry = to_dma_buf_entry_from_kobj(kobj);
> > -     dmabuf = sysfs_entry->dmabuf;
> > -
> > -     if (!dmabuf || !attribute->show)
> > -             return -EIO;
> > -
> > -     return attribute->show(dmabuf, attribute, buf);
> > -}
> > -
> > -static const struct sysfs_ops dma_buf_stats_sysfs_ops = {
> > -     .show = dma_buf_stats_attribute_show,
> > -};
> > -
> > -static ssize_t exporter_name_show(struct dma_buf *dmabuf,
> > -                               struct dma_buf_stats_attribute *attr,
> > -                               char *buf)
> > -{
> > -     return sysfs_emit(buf, "%s\n", dmabuf->exp_name);
> > -}
> > -
> > -static ssize_t size_show(struct dma_buf *dmabuf,
> > -                      struct dma_buf_stats_attribute *attr,
> > -                      char *buf)
> > -{
> > -     return sysfs_emit(buf, "%zu\n", dmabuf->size);
> > -}
> > -
> > -static struct dma_buf_stats_attribute exporter_name_attribute =
> > -     __ATTR_RO(exporter_name);
> > -static struct dma_buf_stats_attribute size_attribute = __ATTR_RO(size);
> > -
> > -static struct attribute *dma_buf_stats_default_attrs[] = {
> > -     &exporter_name_attribute.attr,
> > -     &size_attribute.attr,
> > -     NULL,
> > -};
> > -ATTRIBUTE_GROUPS(dma_buf_stats_default);
> > -
> > -static void dma_buf_sysfs_release(struct kobject *kobj)
> > -{
> > -     struct dma_buf_sysfs_entry *sysfs_entry;
> > -
> > -     sysfs_entry = to_dma_buf_entry_from_kobj(kobj);
> > -     kfree(sysfs_entry);
> > -}
> > -
> > -static const struct kobj_type dma_buf_ktype = {
> > -     .sysfs_ops = &dma_buf_stats_sysfs_ops,
> > -     .release = dma_buf_sysfs_release,
> > -     .default_groups = dma_buf_stats_default_groups,
> > -};
> > -
> > -void dma_buf_stats_teardown(struct dma_buf *dmabuf)
> > -{
> > -     struct dma_buf_sysfs_entry *sysfs_entry;
> > -
> > -     sysfs_entry = dmabuf->sysfs_entry;
> > -     if (!sysfs_entry)
> > -             return;
> > -
> > -     kobject_del(&sysfs_entry->kobj);
> > -     kobject_put(&sysfs_entry->kobj);
> > -}
> > -
> > -
> > -/* Statistics files do not need to send uevents. */
> > -static int dmabuf_sysfs_uevent_filter(const struct kobject *kobj)
> > -{
> > -     return 0;
> > -}
> > -
> > -static const struct kset_uevent_ops dmabuf_sysfs_no_uevent_ops = {
> > -     .filter = dmabuf_sysfs_uevent_filter,
> > -};
> > -
> > -static struct kset *dma_buf_stats_kset;
> > -static struct kset *dma_buf_per_buffer_stats_kset;
> > -int dma_buf_init_sysfs_statistics(void)
> > -{
> > -     dma_buf_stats_kset = kset_create_and_add("dmabuf",
> > -                                              &dmabuf_sysfs_no_uevent_ops,
> > -                                              kernel_kobj);
> > -     if (!dma_buf_stats_kset)
> > -             return -ENOMEM;
> > -
> > -     dma_buf_per_buffer_stats_kset = kset_create_and_add("buffers",
> > -                                                         &dmabuf_sysfs_no_uevent_ops,
> > -                                                         &dma_buf_stats_kset->kobj);
> > -     if (!dma_buf_per_buffer_stats_kset) {
> > -             kset_unregister(dma_buf_stats_kset);
> > -             return -ENOMEM;
> > -     }
> > -
> > -     return 0;
> > -}
> > -
> > -void dma_buf_uninit_sysfs_statistics(void)
> > -{
> > -     kset_unregister(dma_buf_per_buffer_stats_kset);
> > -     kset_unregister(dma_buf_stats_kset);
> > -}
> > -
> > -int dma_buf_stats_setup(struct dma_buf *dmabuf, struct file *file)
> > -{
> > -     struct dma_buf_sysfs_entry *sysfs_entry;
> > -     int ret;
> > -
> > -     if (!dmabuf->exp_name) {
> > -             pr_err("exporter name must not be empty if stats needed\n");
> > -             return -EINVAL;
> > -     }
> > -
> > -     sysfs_entry = kzalloc(sizeof(struct dma_buf_sysfs_entry), GFP_KERNEL);
> > -     if (!sysfs_entry)
> > -             return -ENOMEM;
> > -
> > -     sysfs_entry->kobj.kset = dma_buf_per_buffer_stats_kset;
> > -     sysfs_entry->dmabuf = dmabuf;
> > -
> > -     dmabuf->sysfs_entry = sysfs_entry;
> > -
> > -     /* create the directory for buffer stats */
> > -     ret = kobject_init_and_add(&sysfs_entry->kobj, &dma_buf_ktype, NULL,
> > -                                "%lu", file_inode(file)->i_ino);
> > -     if (ret)
> > -             goto err_sysfs_dmabuf;
> > -
> > -     return 0;
> > -
> > -err_sysfs_dmabuf:
> > -     kobject_put(&sysfs_entry->kobj);
> > -     dmabuf->sysfs_entry = NULL;
> > -     return ret;
> > -}
> > diff --git a/drivers/dma-buf/dma-buf-sysfs-stats.h b/drivers/dma-buf/dma-buf-sysfs-stats.h
> > deleted file mode 100644
> > index 7a8a995b75ba..000000000000
> > --- a/drivers/dma-buf/dma-buf-sysfs-stats.h
> > +++ /dev/null
> > @@ -1,35 +0,0 @@
> > -/* SPDX-License-Identifier: GPL-2.0-only */
> > -/*
> > - * DMA-BUF sysfs statistics.
> > - *
> > - * Copyright (C) 2021 Google LLC.
> > - */
> > -
> > -#ifndef _DMA_BUF_SYSFS_STATS_H
> > -#define _DMA_BUF_SYSFS_STATS_H
> > -
> > -#ifdef CONFIG_DMABUF_SYSFS_STATS
> > -
> > -int dma_buf_init_sysfs_statistics(void);
> > -void dma_buf_uninit_sysfs_statistics(void);
> > -
> > -int dma_buf_stats_setup(struct dma_buf *dmabuf, struct file *file);
> > -
> > -void dma_buf_stats_teardown(struct dma_buf *dmabuf);
> > -#else
> > -
> > -static inline int dma_buf_init_sysfs_statistics(void)
> > -{
> > -     return 0;
> > -}
> > -
> > -static inline void dma_buf_uninit_sysfs_statistics(void) {}
> > -
> > -static inline int dma_buf_stats_setup(struct dma_buf *dmabuf, struct file *file)
> > -{
> > -     return 0;
> > -}
> > -
> > -static inline void dma_buf_stats_teardown(struct dma_buf *dmabuf) {}
> > -#endif
> > -#endif // _DMA_BUF_SYSFS_STATS_H
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index a4d8f2ff94e4..8e23580f1754 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -33,8 +33,6 @@
> >  #include <uapi/linux/dma-buf.h>
> >  #include <uapi/linux/magic.h>
> >
> > -#include "dma-buf-sysfs-stats.h"
> > -
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/dma_buf.h>
> >
> > @@ -184,7 +182,6 @@ static void dma_buf_release(struct dentry *dentry)
> >        */
> >       BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
> >
> > -     dma_buf_stats_teardown(dmabuf);
> >       dmabuf->ops->release(dmabuf);
> >
> >       if (dmabuf->resv == (struct dma_resv *)&dmabuf[1])
> > @@ -765,10 +762,6 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
> >               dmabuf->resv = resv;
> >       }
> >
> > -     ret = dma_buf_stats_setup(dmabuf, file);
> > -     if (ret)
> > -             goto err_dmabuf;
> > -
> >       file->private_data = dmabuf;
> >       file->f_path.dentry->d_fsdata = dmabuf;
> >       dmabuf->file = file;
> > @@ -779,10 +772,6 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
> >
> >       return dmabuf;
> >
> > -err_dmabuf:
> > -     if (!resv)
> > -             dma_resv_fini(dmabuf->resv);
> > -     kfree(dmabuf);
> >  err_file:
> >       fput(file);
> >  err_module:
> > @@ -1802,12 +1791,6 @@ static inline void dma_buf_uninit_debugfs(void)
> >
> >  static int __init dma_buf_init(void)
> >  {
> > -     int ret;
> > -
> > -     ret = dma_buf_init_sysfs_statistics();
> > -     if (ret)
> > -             return ret;
> > -
> >       dma_buf_mnt = kern_mount(&dma_buf_fs_type);
> >       if (IS_ERR(dma_buf_mnt))
> >               return PTR_ERR(dma_buf_mnt);
> > @@ -1821,6 +1804,5 @@ static void __exit dma_buf_deinit(void)
> >  {
> >       dma_buf_uninit_debugfs();
> >       kern_unmount(dma_buf_mnt);
> > -     dma_buf_uninit_sysfs_statistics();
> >  }
> >  __exitcall(dma_buf_deinit);
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > index 0bc492090237..91f4939db89b 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -429,18 +429,6 @@ struct dma_buf {
> >
> >               __poll_t active;
> >       } cb_in, cb_out;
> > -#ifdef CONFIG_DMABUF_SYSFS_STATS
> > -     /**
> > -      * @sysfs_entry:
> > -      *
> > -      * For exposing information about this buffer in sysfs. See also
> > -      * `DMA-BUF statistics`_ for the uapi this enables.
> > -      */
> > -     struct dma_buf_sysfs_entry {
> > -             struct kobject kobj;
> > -             struct dma_buf *dmabuf;
> > -     } *sysfs_entry;
> > -#endif
> >  };
> >
> >  /**
> >
> > base-commit: 26b4309a3ab82a0697751cde52eb336c29c19035
>

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