Re: [PATCH v2 v2 2/2] ext4: allow clearing mballoc stats through mb_stats

liubaolin
2026-04-21T05:23:14Z

Dear all,
I noticed the discussion about where to document the ext4 proc
parameter mb_stats.
I ran:
git grep -n "/proc/fs/ext4" Documentation/
and found that ext4 proc parameters are currently documented in both
Documentation/admin-guide/ext4.rst and
Documentation/filesystems/proc.rst.

To be consistent with the existing documentation, I am thinking
about documenting mb_stats in both places.
If there are no objections, I will send a v3 shortly.
Compared with v2,the v3 patch will add the mb_stats documentation to
both ext4.rst and proc.rst.

Regards,
Baolin

在 2026/4/21 11:40, Ritesh Harjani (IBM) 写道:
> Andreas Dilger <adilger@dilger.ca> writes:
>
>> On Apr 20, 2026, at 03:12, Ojaswin Mujoo <ojaswin@linux.ibm.com> wrote:
>>>
>>> On Sun, Apr 19, 2026 at 02:34:36PM +0800, Baolin Liu wrote:
>>>> From: Baolin Liu <liubaolin@kylinos.cn>
>>>>
>>>> Make /proc/fs/ext4/<dev>/mb_stats writable and clear the runtime
>>>> mballoc statistics when 0 is written.
>>>>
>>>> Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
>>>> ---
>>> Hi Baolin, thanks for the changes.
>>>
>>> Seems like userspace doesn't have any way to know that writing 0 will
>>> clear the that. Well, I guess if you are looking at this file you are
>>> anyways debugging kernel code so that should be fine
>>
>> That could be documented in Documentation/filesystems/ext4/allocators.rst,
>> or better would be to add a new file that covers mballoc in more detail.
>>
>
> I started looking for ext4's control knobs for sys-admins in kernel
> Documentation where we should ideally document this, and I see those
> are declared here..
>
> Documentation/admin-guide/ext4.rst
> Documentation/ABI/testing/sysfs-fs-ext4.rst
>
> Looking at this and the relevant code, I see all /proc/ entries in ext4
> are all readable and sysfs entries for ext4 are mostly the control knobs
> which are declared in above admin guide.
>
> But now this patch adds a control knob to /proc/fs/ext4/<dev>/mb_stats,
> to clear the stats :).
>
> I guess we could have simply documented a new control knob value (e.g.
> "2") for clearing the stats via /sys/fs/ext4/<dev>/mb_stats itself or
> maybe even having mb_stats_clear file in sysfs wasn't bad either... But
> either ways, clearing the stats via the same procfs mb_stats file is not
> totally bad and I don't have a strong preference.
>
>
> For documenting this, we can add mb_stats entry under /proc section in
> Documentation/admin-guide/ext4.rst and document this change. Something
> like -
>
> mb_stats
> reports runtime statistics from multiblock allocator (mballoc),
> including allocation request counts, groups scanned,
> per-criteria scan hits (cr_p2_aligned, cr_goal_fast,
> cr_best_avail, cr_goal_slow, cr_any_free), groups / extents
> scanned, goal hits, buddy bitmap generations, and preallocation
> usage etc.
> Writing 0 to this procfs file resets all counters to zero.
>
>
> -ritesh