On Fri, Feb 26, 2021 at 09:35:09AM +0100, Michal Hocko wrote: > I think it would be helpful to call out that specific case explicitly > here. I can see only one scenario (are there more?) > __free_huge_page() isolate_or_dissolve_huge_page > PageHuge() == T > alloc_and_dissolve_huge_page > alloc_fresh_huge_page() > spin_lock(hugetlb_lock) > // PageHuge() && !PageHugeFreed && > // !PageCount() > spin_unlock(hugetlb_lock) > spin_lock(hugetlb_lock) > 1) update_and_free_page > PageHuge() == F > __free_pages() > 2) enqueue_huge_page > SetPageHugeFreed() > spin_unlock(&hugetlb_lock)
I do not think there are more scenarios. The thing is to find a !page_count && !PageHugeFreed. AFAICS, this can only happen after: put_page->put_page_test_zero->..->_free_huge_page gets called but __free_huge_page has not reached enqueue_huge_page yet (as you just described above) By calling out this case, you meant to describe it in the changelog? > > > In this case we retry as the window race is quite small and we have high > > chances to succeed next time. > > > > With regard to the allocation, we restrict it to the node the page belongs > > to with __GFP_THISNODE, meaning we do not fallback on other node's zones. > > > > Note that gigantic hugetlb pages are fenced off since there is a cyclic > > dependency between them and alloc_contig_range. > > > > Signed-off-by: Oscar Salvador <[email protected]> > > Thanks this looks much better than the initial version. One nit below. > Acked-by: Michal Hocko <[email protected]> Thanks! > > +bool isolate_or_dissolve_huge_page(struct page *page) > > +{ > > + struct hstate *h = NULL; > > + struct page *head; > > + bool ret = false; > > + > > + spin_lock(&hugetlb_lock); > > + if (PageHuge(page)) { > > + head = compound_head(page); > > + h = page_hstate(head); > > + } > > + spin_unlock(&hugetlb_lock); > > + > > + /* > > + * The page might have been dissolved from under our feet. > > + * If that is the case, return success as if we dissolved it ourselves. > > + */ > > + if (!h) > > + return true; > > I am still fighting with this construct a bit. It is not really clear > what the lock is protecting us from here. alloc_fresh_huge_page deals > with all potential races and this looks like an optimistic check to save > some work. But in fact the lock is really necessary for correctness > because hstate might be completely bogus without the lock or us holding > a reference on the poage. The following construct would be more > explicit and compact. What do you think? > > struct hstate *h; > > /* > * The page might have been dissloved from under our feet > * so make sure to carefully check the state under the lock. > * Return success on when racing as if we dissloved the page > * ourselves. > */ > spin_lock(&hugetlb_lock); > if (PageHuge(page)) { > head = compound_head(page); > h = page_hstate(head); > } else { > spin_unlock(&hugetlb_lock); > return true; > } > spin_unlock(&hugetlb_lock); Yes, I find this less eyesore. I will fix it up in v4. -- Oscar Salvador SUSE L3
