On Wed, Aug 19, 2020 at 4:43 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Tue, Aug 18, 2020 at 09:27:05PM -0700, Alexander Duyck wrote: > > + /* > > + * Page is compound. We know the order before we know if it is > > + * on the LRU so we cannot assume it is THP. However since the > > + * page will have the LRU validated shortly we can use the value > > + * to skip over this page for now or validate the LRU is set and > > + * then isolate the entire compound page if we are isolating to > > + * generate a CMA page. > > + */ > > + if (PageCompound(page)) { > > + const unsigned int order = compound_order(page); > > + > > + if (likely(order < MAX_ORDER)) > > + low_pfn += (1UL << order) - 1; > > Hmm. You're checking for PageCompound but then skipping 1UL << order. > That only works if PageHead. If instead this is PageCompound because > it's PageTail, you need to do something like: > > low_pfn |= (1UL << order) - 1; > > which will move you to the end of the page you're in the middle of. Can you successfully call get_page_unless_zero in a tail page? I thought their reference count was 0? There is a get_page_unless_zero call before the PageCompound check, so I don't think we can get a tail page. > If PageTail can't actually happen here, then it's better to check for > PageHead explicitly and WARN_ON if you get a PageTail (eg a page was > combined into a compound page after you processed the earlier head page). > > Is it possible the page you've found is hugetlbfs? Those can have orders > larger than MAX_ORDER. So in theory we only need to jump pageblock_order. However there are some architectures where that is not a fixed constant and so it would have some additional overhead if I am not mistaken. In addition we should have been only provided a pageblock if i am not mistaken so the check further down that prevents low_pfn from passing end_pfn should reset to the correct value.