On Tue, Jan 12, 2021 at 8:37 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Tue 12-01-21 19:43:21, Muchun Song wrote: > > On Tue, Jan 12, 2021 at 7:17 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Tue 12-01-21 18:13:02, Muchun Song wrote: > > > > On Tue, Jan 12, 2021 at 6:02 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > On Sun 10-01-21 20:40:14, Muchun Song wrote: > > > > > [...] > > > > > > @@ -1770,6 +1788,14 @@ int dissolve_free_huge_page(struct page *page) > > > > > > int nid = page_to_nid(head); > > > > > > if (h->free_huge_pages - h->resv_huge_pages == 0) > > > > > > goto out; > > > > > > + > > > > > > + /* > > > > > > + * We should make sure that the page is already on the free list > > > > > > + * when it is dissolved. > > > > > > + */ > > > > > > + if (unlikely(!PageHugeFreed(head))) > > > > > > + goto out; > > > > > > + > > > > > > > > > > Do you really want to report EBUSY in this case? This doesn't make much > > > > > sense to me TBH. I believe you want to return 0 same as when you race > > > > > and the page is no longer PageHuge. > > > > > > > > Return 0 is wrong. Because the page is not freed to the buddy allocator. > > > > IIUC, dissolve_free_huge_page returns 0 when the page is already freed > > > > to the buddy allocator. Right? > > > > > > 0 is return when the page is either dissolved or it doesn't need > > > dissolving. If there is a race with somebody else freeing the page then > > > there is nothing to dissolve. Under which condition it makes sense to > > > report the failure and/or retry dissolving? > > > > If there is a race with somebody else freeing the page, the page > > can be freed to the hugepage pool not the buddy allocator. Do > > you think that this page is dissolved? > > OK, I see what you mean. Effectively the page would be in a limbo, not > yet in the pool nor in the allocator but it can find its way to the > either of the two. But I still dislike returning a failure because that > would mean e.g. memory hotplug to fail. Can you simply retry inside this > code path (drop the lock, cond_resched and retry)? Yeah. This is what I want to do (making the memory hotplug as successful as possible). So I send the patch: [PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page Adding a simple retry inside this function when hitting this race is also fine to me. I can do that. > -- > Michal Hocko > SUSE Labs
