On 1/7/21 7:11 AM, Muchun Song wrote: > 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. You are correct. This is also an issue/potential problem with this race. It seems that adding the state information might be the least complex way to address issue. -- Mike Kravetz
