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

[Kamlesh Gurudasani]

Waiman Long <longman@xxxxxxxxxx> 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@xxxxxx>
>> ---
>>   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@xxxxxxxxxx>
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.

Cheers,
Kamlesh