Re: [EXTERNAL] Re: [PATCH] padata: Honor the caller's alignment in case of chunk_size 0

[Waiman Long <longman@redhat.com>]

On 8/25/24 07:34, Kamlesh Gurudasani wrote:
> Waiman Long <longman@redhat.com> writes:
>
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>> On 8/21/24 17:02, Kamlesh Gurudasani wrote:
>>> In the case where we are forcing the ps.chunk_size to be at least 1,
>>> we are ignoring the caller's alignment.
>>>
>>> Move the forcing of ps.chunk_size to be at least 1 before rounding it
>>> up to caller's alignment, so that caller's alignment is honored.
>>>
>>> While at it, use max() to force the ps.chunk_size to be at least 1 to
>>> improve readability.
>>>
>>> Fixes: 6d45e1c948a8 ("padata: Fix possible divide-by-0 panic in padata_mt_helper()")
>>> Signed-off-by: Kamlesh Gurudasani <kamlesh@ti.com>
>>> ---
>>>    kernel/padata.c | 12 ++++--------
>>>    1 file changed, 4 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/kernel/padata.c b/kernel/padata.c
>>> index 0fa6c2895460..d8a51eff1581 100644
>>> --- a/kernel/padata.c
>>> +++ b/kernel/padata.c
>>> @@ -509,21 +509,17 @@ void __init padata_do_multithreaded(struct padata_mt_job *job)
>>>    
>>>    	/*
>>>    	 * Chunk size is the amount of work a helper does per call to the
>>> -	 * thread function.  Load balance large jobs between threads by
>>> +	 * thread function. Load balance large jobs between threads by
>>>    	 * increasing the number of chunks, guarantee at least the minimum
>>>    	 * chunk size from the caller, and honor the caller's alignment.
>>> +	 * Ensure chunk_size is at least 1 to prevent divide-by-0
>>> +	 * panic in padata_mt_helper().
>>>    	 */
>>>    	ps.chunk_size = job->size / (ps.nworks * load_balance_factor);
>>>    	ps.chunk_size = max(ps.chunk_size, job->min_chunk);
>>> +	ps.chunk_size = max(ps.chunk_size, 1ul);
>>>    	ps.chunk_size = roundup(ps.chunk_size, job->align);
>>>    
>>> -	/*
>>> -	 * chunk_size can be 0 if the caller sets min_chunk to 0. So force it
>>> -	 * to at least 1 to prevent divide-by-0 panic in padata_mt_helper().`
>>> -	 */
>>> -	if (!ps.chunk_size)
>>> -		ps.chunk_size = 1U;
>>> -
>>>    	list_for_each_entry(pw, &works, pw_list)
>>>    		if (job->numa_aware) {
>>>    			int old_node = atomic_read(&last_used_nid);
>>>
>>> ---
>>> base-commit: b311c1b497e51a628aa89e7cb954481e5f9dced2
>>> change-id: 20240822-max-93c17adc6457
>> LGTM, my only nit is the use of "1ul" which is less common and harder to
>> read than "1UL" as the former one may be misread as a "lul" variable.
>>
>> Acked-by:  Waiman Long <longman@redhat.com>
> Thanks for the Acked-by, Waiman. I understand your point, though Daniel seems
> to be okay with this, so will keep it as is this time.

This is just a suggestion in case you need to update your patch. I am 
fine with keeping it as is if no further update is needed.

Cheers,
Longman