On Thu, Jan 7, 2021 at 10:11 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Thu 07-01-21 20:59:33, Muchun Song wrote: > > On Thu, Jan 7, 2021 at 8:38 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > [...] > > > Right. Can we simply back off in the dissolving path when ref count is > > > 0 && PageHuge() if list_empty(page->lru)? Is there any other scenario > > > when the all above is true and the page is not being freed? > > > > The list_empty(&page->lru) may always return false. > > The page before freeing is on the active list > > (hstate->hugepage_activelist).Then it is on the free list > > after freeing. So list_empty(&page->lru) is always false. > > The point I was trying to make is that the page has to be enqueued when > it is dissolved and freed. If the page is not enqueued then something > racing. But then I have realized that this is not a great check to > detect the race because pages are going to be released to buddy > allocator and that will reuse page->lru again. So scratch that and sorry > for the detour. > > But that made me think some more and one way to reliably detect the race > should be PageHuge() check in the freeing path. This is what dissolve > path does already. PageHuge becomes false during update_and_free_page() > while holding the hugetlb_lock. So can we use that? It may make the thing complex. Apart from freeing it to the buddy allocator, free_huge_page also does something else for us. If we detect the race in the freeing path, if it is not a HugeTLB page, the freeing path just returns. We also should move those things to the dissolve path. Right? But I find a tricky problem to solve. See free_huge_page(). If we are in non-task context, we should schedule a work to free the page. We reuse the page->mapping. If the page is already freed by the dissolve path. We should not touch the page->mapping. So we need to check PageHuge(). The check and llist_add() should be protected by hugetlb_lock. But we cannot do that. Right? If dissolve happens after it is linked to the list. We also should remove it from the list (hpage_freelist). It seems to make the thing more complex. > -- > Michal Hocko > SUSE Labs
