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

[Muchun Song]

On Mon, Nov 23, 2020 at 6:43 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 23-11-20 18:36:33, Muchun Song wrote:
> > On Mon, Nov 23, 2020 at 5:43 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Mon 23-11-20 16:53:53, Muchun Song wrote:
> > > > On Mon, Nov 23, 2020 at 3:40 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > >
> > > > > On Fri 20-11-20 23:44:26, Muchun Song wrote:
> > > > > > On Fri, Nov 20, 2020 at 9:11 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Fri 20-11-20 20:40:46, Muchun Song wrote:
> > > > > > > > On Fri, Nov 20, 2020 at 4:42 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > > > On Fri 20-11-20 14:43:04, Muchun Song wrote:
> > > > > > > > > [...]
> > > > > > > > >
> > > > > > > > > Thanks for improving the cover letter and providing some numbers. I have
> > > > > > > > > only glanced through the patchset because I didn't really have more time
> > > > > > > > > to dive depply into them.
> > > > > > > > >
> > > > > > > > > Overall it looks promissing. To summarize. I would prefer to not have
> > > > > > > > > the feature enablement controlled by compile time option and the kernel
> > > > > > > > > command line option should be opt-in. I also do not like that freeing
> > > > > > > > > the pool can trigger the oom killer or even shut the system down if no
> > > > > > > > > oom victim is eligible.
> > > > > > > >
> > > > > > > > Hi Michal,
> > > > > > > >
> > > > > > > > I have replied to you about those questions on the other mail thread.
> > > > > > > >
> > > > > > > > Thanks.
> > > > > > > >
> > > > > > > > >
> > > > > > > > > One thing that I didn't really get to think hard about is what is the
> > > > > > > > > effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be
> > > > > > > > > invalid when racing with the split. How do we enforce that this won't
> > > > > > > > > blow up?
> > > > > > > >
> > > > > > > > This feature depends on the CONFIG_SPARSEMEM_VMEMMAP,
> > > > > > > > in this case, the pfn_to_page can work. The return value of the
> > > > > > > > pfn_to_page is actually the address of it's struct page struct.
> > > > > > > > I can not figure out where the problem is. Can you describe the
> > > > > > > > problem in detail please? Thanks.
> > > > > > >
> > > > > > > struct page returned by pfn_to_page might get invalid right when it is
> > > > > > > returned because vmemmap could get freed up and the respective memory
> > > > > > > released to the page allocator and reused for something else. See?
> > > > > >
> > > > > > If the HugeTLB page is already allocated from the buddy allocator,
> > > > > > the struct page of the HugeTLB can be freed? Does this exist?
> > > > >
> > > > > Nope, struct pages only ever get deallocated when the respective memory
> > > > > (they describe) is hotremoved via hotplug.
> > > > >
> > > > > > If yes, how to free the HugeTLB page to the buddy allocator
> > > > > > (cannot access the struct page)?
> > > > >
> > > > > But I do not follow how that relates to my concern above.
> > > >
> > > > Sorry. I shouldn't understand your concerns.
> > > >
> > > > vmemmap pages                 page frame
> > > > +-----------+   mapping to   +-----------+
> > > > |           | -------------> |     0     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     1     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     2     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     3     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     4     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     5     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     6     |
> > > > +-----------+                +-----------+
> > > > |           | -------------> |     7     |
> > > > +-----------+                +-----------+
> > > >
> > > > In this patch series, we will free the page frame 2-7 to the
> > > > buddy allocator. You mean that pfn_to_page can return invalid
> > > > value when the pfn is the page frame 2-7? Thanks.
> > >
> > > 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?

>
> struct page *page = pfn_to_page();
> // remapping happens here
> // page content is no longer valid because its backing memory can be

If we only read the page->compound_head. The content is
also valid. Because the value of compound_head is the same
for the tail page struct of HugeTLB page.

> // reused for whatever purpose.

> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun