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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

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