Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page

[Michal Hocko]

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