On Wed 06-01-21 13:07:40, Mike Kravetz wrote: > On 1/6/21 12:02 PM, Michal Hocko wrote: > > On Wed 06-01-21 11:30:25, Mike Kravetz wrote: > >> On 1/6/21 8:35 AM, Michal Hocko wrote: > >>> On Wed 06-01-21 16:47:35, Muchun Song wrote: > >>>> Because we only can isolate a active page via isolate_huge_page() > >>>> and hugetlbfs_fallocate() forget to mark it as active, we cannot > >>>> isolate and migrate those pages. > >>> > >>> I've little bit hard time to understand this initially and had to dive > >>> into the code to make sense of it. I would consider the following > >>> wording easier to grasp. Feel free to reuse if you like. > >>> " > >>> If a new hugetlb page is allocated during fallocate it will not be > >>> marked as active (set_page_huge_active) which will result in a later > >>> isolate_huge_page failure when the page migration code would like to > >>> move that page. Such a failure would be unexpected and wrong. > >>> " > >>> > >>> Now to the fix. I believe that this patch shows that the > >>> set_page_huge_active is just too subtle. Is there any reason why we > >>> cannot make all freshly allocated huge pages active by default? > >> > >> I looked into that yesterday. The primary issue is in page fault code, > >> hugetlb_no_page is an example. If page_huge_active is set, then it can > >> be isolated for migration. So, migration could race with the page fault > >> and the page could be migrated before being added to the page table of > >> the faulting task. This was an issue when hugetlb_no_page set_page_huge_active > >> right after allocating and clearing the huge page. Commit cb6acd01e2e4 > >> moved the set_page_huge_active after adding the page to the page table > >> to address this issue. > > > > Thanks for the clarification. I was not aware of this subtlety. The > > existing comment is not helping much TBH. I am still digesting the > > suggested race. The page is new and exclusive and not visible via page > > tables yet, so the only source of the migration would be pfn based > > (hotplug, poisoning), right? > > That is correct. > > > > Btw. s@set_page_huge_active@set_page_huge_migrateable@ would help > > readability IMHO. With a comment explaining that this _has_ to be called > > after the page is fully initialized. > > Agree, I will add that as a future enhancement. Thanks! -- Michal Hocko SUSE Labs
