Re: [QUESTIONS] THP allocation in NUMA fault migration path

[Yang Shi <yang.shi@linux.alibaba.com>]

On 4/17/19 11:32 PM, Michal Hocko wrote:
> On Wed 17-04-19 21:15:41, Yang Shi wrote:
>> Hi folks,
>>
>>
>> I noticed that there might be new THP allocation in NUMA fault migration
>> path (migrate_misplaced_transhuge_page()) even when THP is disabled (set to
>> "never"). When THP is set to "never", there should be not any new THP
>> allocation, but the migration path is kind of special. So I'm not quite sure
>> if this is the expected behavior or not?
>>
>>
>> And, it looks this allocation disregards defrag setting too, is this
>> expected behavior too?H
> Could you point to the specific code? But in general the miTgration path

Yes. The code is in migrate_misplaced_transhuge_page() called by 
do_huge_pmd_numa_page().

It would just do:
alloc_pages_node(node, (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE), 
HPAGE_PMD_ORDER);
without checking if transparent_hugepage is enabled or not.

THP may be disabled before calling into do_huge_pmd_numa_page(). The 
do_huge_pmd_wp_page() does check if THP is disabled or not. If THP is 
disabled, it just tries to allocate 512 base pages.

> should allocate the memory matching the migration origin. If the origin
> was a THP then I find it quite natural if the target was a huge page as

Yes, this is what I would like to confirm. Migration allocates a new THP 
to replace the old one.

> well. How hard the allocation should try is another question and I
> suspect we do want to obedy the defrag setting.

Yes, I thought so too. However, THP NUMA migration was added in 3.8 by 
commit b32967f ("mm: numa: Add THP migration for the NUMA working set 
scanning fault case."). It disregarded defrag setting at the very 
beginning. So, I'm not quite sure if it was done on purpose or just 
forgot it.

Thanks,
Yang