Re: [RFC PATCH v1.1 00/11] mm/damon/sysfs: kobject_del() directories that users can create/remove

[SeongJae Park <sj@kernel.org>]

On Wed, 17 Jun 2026 07:47:54 -0700 SeongJae Park <sj@kernel.org> wrote:

> DAMON sysfs interface allows users to create and remove arbitrary number
> of directories on sysfs, using a few files having 'nr_' prefix.  For
> example, 'nr_kdamonds'.  When the user writes a number 'N' to the files,
> directories having name starting from '0' to 'N - 1' are created in the
> same directory.  The pre-existing number-named directories are removed
> before creating the new directories.
> 
> For the removal of the existing directories, DAMON sysfs interface use
> only kobject_put().  Because DAMON sysfs interface is the only kernel
> component that manages the directories, there is no problem in normal
> situations.  However, if CONFIG_DEBUG_KOBJECT_RELEASE is enabled, the
> removal of dirs are delayed.  Let's suppose a user writes a non-zero
> number to the 'nr_*' files while there are pre-existing number-named
> directories, on the config enabled kernel.  DAMON sysfs interface
> decreases the reference counts of the existing directories and
> immediately creates new directories.  Because the removal of the sysfs
> directories is delayed, it shows some pre-existing directories of the
> same names when it tries to create the new directories, and fails.
> 
> For example, the issue can be triggered like below:
> 
>     # grep DEBUG_KOBJECT_RELEASE /boot/config-$(uname -r)
>     CONFIG_DEBUG_KOBJECT_RELEASE=y
>     # ls
>     nr_kdamonds
>     # echo 1 > nr_kdamonds
>     # echo 1 > nr_kdamonds
>     bash: echo: write error: File exists
>     # dmesg
>     [...]
>     [  300.880458] kobject: kobject_add_internal failed for 0 with -EEXIST, don't try to register things with the same name in the same directory.
>     [...]
> 
> Some of the error handling paths of the directories also lack the
> kobject_del() call.  If the user uses nr_* file right after the errors,
> similar issues can happen.

Sashiko found [1] one more issue of the same class in
damon_sysfs_schemes_add_dirs().  I will fix it in the next spin.

[1] https://lore.kernel.org/20260618004428.82261-1-sj@kernel.org


Thanks,
SJ

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