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

[Michal Hocko]

On Mon 23-11-20 12:45:13, Matthew Wilcox wrote:
> 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 // page content is no longer valid because its backing memory can be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().

You are right. I have managed to completely confuse myself. Sorry about
the noise!

-- 
Michal Hocko
SUSE Labs