Re: [PATCH v2 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/24/2022 3:24 PM, Alistair Popple wrote:
> 
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> On 10/24/2022 10:36 AM, Alistair Popple wrote:
>>> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
>>>
>>>> When THP migration, if THPs are split and all subpages are migrated successfully
>>>> , the migrate_pages() will still return the number of THP that were not migrated.
>>>> That will confuse the callers of migrate_pages(), for example, which will make
>>>> the longterm pinning failed though all pages are migrated successfully.
>>>>
>>>> Thus we should return 0 to indicate all pages are migrated in this case.
>>>>
>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> ---
>>>> Changes from v1:
>>>> - Fix the return value of migrate_pages() instead of fixing the
>>>>     callers' validation.
>>>> ---
>>>>    mm/migrate.c | 7 +++++++
>>>>    1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 8e5eb6e..1da0dbc 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1582,6 +1582,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>>>    	 */
>>>>    	list_splice(&ret_pages, from);
>>>>
>>>> +	/*
>>>> +	 * Return 0 in case all subpages of fail-to-migrate THPs are
>>>> +	 * migrated successfully.
>>>> +	 */
>>>> +	if (nr_thp_split && list_empty(from))
>>>> +		rc = 0;
>>> Why do you need to check nr_thp_split? Wouldn't list_empty(from) == True
>>
>> Only in the case of THP split, we can meet this abnormal case. So if no THP
>> split, just return the original 'rc' instead of validating the list, since the
>> 'nr_thp_split' validation is cheaper than the list_empty() validation IMHO.
> 
> Is it really that much cheaper? We're already retrying migrations
> multiple times, etc. so surely the difference here would be marginal at
> best, and IMHO the code would be much clearer if we always set rc = 0
> when list_empty(from) = True.

Yeah, the difference is marginal and I have no strong preference. OK, 
will drop the 'nr_thp_split' in next version. Thanks.

>>> imply success? And if it doesn't imply success wouldn't it be possible
>>> to end up with nr_thp_split && list_empty(from) whilst still having
>>> pages that failed to migrate?
>>> The list management and return code logic from unmap_and_move() has
>>> gotten pretty difficult to follow and could do with some rework IMHO.
>>
>> Yes, Huang Ying has sent a RFC patchset[1] doing some code refactor, which seems
>> a good start.
> 
> Thanks for pointing that out, I looked at it a while back but missed the
> clean ups. I was kind of waiting for the non-RFC version before taking
> another closer look.
> 
>> [1] https://lore.kernel.org/all/20220921060616.73086-1-ying.huang@intel.com/