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

[Muchun Song]

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.

>
> --
> Michal Hocko
> SUSE Labs



--
Yours,
Muchun