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

[Matthew Wilcox]

On Mon, Nov 23, 2020 at 11:42:58AM +0100, Michal Hocko wrote:
> On Mon 23-11-20 18:36:33, Muchun Song 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 
> 
> struct page *page = pfn_to_page();
> // remapping happens here
> // page content is no longer valid because its backing memory can be
> // reused for whatever purpose.

pfn_to_page() returns you a virtual address.  That virtual address
remains a valid pointer to exactly the same contents, it's just that
the page tables change to point to a different struct page which has
the same compound_head().