On Mon 23-11-20 20:40:40, Muchun Song wrote: > On Mon, Nov 23, 2020 at 8:18 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Mon 23-11-20 20:07:23, Muchun Song wrote: > > > On Mon, Nov 23, 2020 at 7:32 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > [...] > > > > > > > > No I really mean that pfn_to_page will give you a struct page pointer > > > > > > > > from pages which you release from the vmemmap page tables. Those pages > > > > > > > > might get reused as soon sa they are freed to the page allocator. > > > > > > > > > > > > > > We will remap vmemmap pages 2-7 (virtual addresses) to page > > > > > > > frame 1. And then we free page frame 2-7 to the buddy allocator. > > > > > > > > > > > > And this doesn't really happen in an atomic fashion from the pfn walker > > > > > > POV, right? So it is very well possible that > > > > > > > > > > Yeah, you are right. But it may not be a problem for HugeTLB pages. > > > > > Because in most cases, we only read the tail struct page and get the > > > > > head struct page through compound_head() when the pfn is within > > > > > a HugeTLB range. Right? > > > > > > > > Many pfn walkers would encounter the head page first and then skip over > > > > the rest. Those should be reasonably safe. But there is no guarantee and > > > > the fact that you need a valid page->compound_head which might get > > > > scribbled over once you have the struct page makes this extremely > > > > subtle. > > > > > > In this patch series, we can guarantee that the page->compound_head > > > is always valid. Because we reuse the first tail page. Maybe you need to > > > look closer at this series. Thanks. > > > > I must be really terrible exaplaining my concern. Let me try one last > > time. It is really _irrelevant_ what you do with tail pages. The > > underlying problem is that you are changing struct pages under users > > without any synchronization. What used to be a valid struct page will > > turn into garbage as soon as you remap vmemmap page tables. > > Thank you very much for your patient explanation. So if the pfn walkers > always try get the head struct page through compound_head() when it > encounter a tail struct page. There will be no concerns. Do you agree? No, I do not agree. Please read again. The content of the struct page might be a complete garbage at any time after pfn_to_page returns a struct page. So there is no valid compound_head anywamore. -- Michal Hocko SUSE Labs
