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

From: Yang Shi
Date: Thu Apr 18 2019 - 12:18:28 EST

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:
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.