Re: [PATCH v3 v3 2/2] ext4: allow clearing mballoc stats through mb_stats

[Baokun Li <libaokun@linux.alibaba.com>]

On 2026/4/24 16:09, liubaolin wrote:
>
>
> 在 2026/4/24 0:19, Theodore Tso 写道:
>> On Wed, Apr 22, 2026 at 09:50:25AM +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.
>>
>> At the moment to enable mb_stats the system administrator needs to
>> write "1" to /sys/fs/ext4/<dev>/mb_stats, and writing "0" to the sysfs
>> file will pauce the statistics colleciton (but not clear the
>> statistics).  Adding a way to clear the statistics by writing to the
>> procfs file might be confusing to users.
>>
>> So.... as a suggestion, if you're adding to the ability to write to
>> /proc/fs/.../mb_stats, what if we make things work by
>>
>>     * Write 1 to /proc/fs/.../mb_stats to  enable statistics collection
>>     * Write 0 to /proc/fs/.../mb_stats to  disable statistics collection
>>     * Write -1 to /proc/fs/.../mb_stats to clear statistics counters
>>
>> And then deprecate the /sys/fs/.../mb_stats variable (but we probably
>> won't be able to remove it for at least a year or two).
>>
>>                                         - Ted
> Dear Ted, Baokun,
>    Thank you for your review and suggestions.
>    Since you mentioned that /sys/fs/.../mb_stats cannot be deleted in
> the short term,
>    I plan to modify and submit a v4 patch according to the following
> strategy.
>
>    1. Change `/proc/fs/.../mb_stats` to read-write mode.
>     * Read `/proc/fs/.../mb_stats` to show statistics counters.
>     * Write 0 to `/proc/fs/.../mb_stats` to disable statistics
> collection.
>     * Write 1 to `/proc/fs/.../mb_stats` to enable statistics collection.
>     * Write 2 to `/proc/fs/.../mb_stats` to clear statistics counters.
>
>    2. Do not delete the `/sys/fs/.../mb_stats` node for now; implement
> the same write control logic.
>     * Write 0 to `/sys/fs/.../mb_stats` to disable statistics collection.
>     * Write 1 to `/sys/fs/.../mb_stats` to enable statistics collection.
>     * Write 2 to `/sys/fs/.../mb_stats` to clear statistics counters.
>
>     Delete `/sys/fs/.../mb_stats` later when it is possible to delete it.
>
>    3. Modify the relevant documentation for `mb_stats`.
>     Documentation/ABI/testing/sysfs-fs-ext4
>     Documentation/admin-guide/ext4.rst
>     Documentation/filesystems/proc.rst
>
>    Compared to your suggestion, I recommend using the value 2 for the
> clear operation because s_mb_stats is an unsigned int variable, and
> using -1 requires changing the variable type.
>    I suggest avoiding changing the s_mb_stats variable type unless
> absolutely necessary.
>
>    Do you think this modification is appropriate?
>    If there are no problems, I will start modifying the code and
> submit the v4 patch as soon as possible.

For the clear command, we only handle it without storing it, so s_mb_stats
remains unchanged and still stores only 0 and non-zero values to represent
disabled and enabled, respectively. Otherwise, you will have to deal with
a large number of s_mb_stats checks

That means the /sys/fs/.../mb_stats interface does not need to support
clearing, but it might make sense to add a deprecation warning there.

Then in `/proc/fs/.../mb_stats`, writing 0 or a positive number passes
it to s_mb_stats, writing -1 performs a reset, and other negative values
return -EINVAL.


Cheers,
Baokun