On 11/23/2017 04:11 AM, Mike Kravetz wrote: > On 11/22/2017 07:28 AM, Michal Hocko wrote: >> Hi, >> is there any reason why we enforce the overcommit limit during hugetlb >> pages migration? It's in alloc_huge_page_node->__alloc_buddy_huge_page >> path. I am wondering whether this is really an intentional behavior. > > I do not think it was intentional. But, I was not around when that > code was added. > >> The page migration allocates a page just temporarily so we should be >> able to go over the overcommit limit for the migration duration. The >> reason I am asking is that hugetlb pages tend to be utilized usually >> (otherwise the memory would be just wasted and pool shrunk) but then >> the migration simply fails which breaks memory hotplug and other >> migration dependent functionality which is quite suboptimal. You can >> workaround that by increasing the overcommit limit. > > Yes. In an environment making optimal use of huge pages, you are unlikely > to have 'spare pages' set aside for a potential migration operation. So > I agree that it would make sense to try and allocate overcommit pages for > this purpose.
Thank you for pointing this out, Michal, Mike. Doing overcommitting in hugepage migration is totally right to me, I just didn't notice it when I wrote the code. > >> Why don't we simply migrate as long as we are able to allocate the >> target hugetlb page? I have a half baked patch to remove this >> restriction, would there be an opposition to do something like that? > > I would not be opposed and would help with this effort. My concern would > be any subtle hugetlb accounting issues once you start messing with > additional overcommit pages. Yes, hugetlb accounting always needs care when touching related code. I can help testing. Thanks, Naoya Horiguchi
