Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Feb 23, 2021 at 5:28 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Feb 22, 2021 at 04:00:27PM -0800, Mike Kravetz wrote:
> > > -static void update_and_free_page(struct hstate *h, struct page *page)
> > > +static int update_and_free_page(struct hstate *h, struct page *page)
> > > +   __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> > >  {
> > >     int i;
> > > +   int nid = page_to_nid(page);
> > >
> > >     if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> > > -           return;
> > > +           return 0;
> > >
> > >     h->nr_huge_pages--;
> > > -   h->nr_huge_pages_node[page_to_nid(page)]--;
> > > +   h->nr_huge_pages_node[nid]--;
> > > +   VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> > > +   VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> > > +   set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > > +   set_page_refcounted(page);
> >
> > I think you added the set_page_refcounted() because the huge page will
> > appear as just a compound page without a reference after dropping the
> > hugetlb lock?  It might be better to set the reference before modifying
> > the destructor.  Otherwise, page scanning code could find the non-hugetlb
> > compound page with no reference.  I could not find any code where this
> > would be a problem, but I think it would be safer to set the reference
> > first.
>
> But we already had set_page_refcounted() before this patchset there.
> Are the worries only because we drop the lock? AFAICS, the "page-scanning"
> problem could have happened before as well?
> Although, what does page scanning mean in this context?
>
> I am not opposed to move it above, but I would like to understand the concern
> here.
>
> >
> > > +   spin_unlock(&hugetlb_lock);
> >
> > I really like the way this code is structured.  It is much simpler than
> > previous versions with retries or workqueue.  There is nothing wrong with
> > always dropping the lock here.  However, I wonder if we should think about
> > optimizing for the case where this feature is not enabled and we are not
> > freeing a 1G huge page.  I suspect this will be the most common case for
> > some time, and there is no need to drop the lock in this case.
> >
> > Please do not change the code based on my comment.  I just wanted to bring
> > this up for thought.
> >
> > Is it as simple as checking?
> >         if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> >                 spin_unlock(&hugetlb_lock);
> >
> >         /* before return */
> >         if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> >                 spin_lock(&hugetlb_lock);
>
> AFAIK, we at least need the hstate_is_gigantic? Comment below says that
> free_gigantic_page might block, so we need to drop the lock.
> And I am fine with the change overall.
>
> Unless I am missing something, we should not need to drop the lock unless
> we need to allocate vmemmap pages (apart from gigantic pages).
>
> >
> > > +
> > > +   if (alloc_huge_page_vmemmap(h, page)) {
> > > +           int zeroed;
> > > +
> > > +           spin_lock(&hugetlb_lock);
> > > +           INIT_LIST_HEAD(&page->lru);
> > > +           set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > > +           h->nr_huge_pages++;
> > > +           h->nr_huge_pages_node[nid]++;
>
> I think prep_new_huge_page() does this for us?

Actually, there are some differences. e.g. prep_new_huge_page()
will reset hugetlb cgroup and ClearHPageFreed, but we do not need
them here. And prep_new_huge_page will acquire and release
the hugetlb_lock. But here we also need hold the lock to update
the surplus counter and enqueue the page to the free list.
So I do not think reuse prep_new_huge_page is a good idea.

>
> > > +
> > > +           /*
> > > +            * If we cannot allocate vmemmap pages, just refuse to free the
> > > +            * page and put the page back on the hugetlb free list and treat
> > > +            * as a surplus page.
> > > +            */
> > > +           h->surplus_huge_pages++;
> > > +           h->surplus_huge_pages_node[nid]++;
> > > +
> > > +           /*
> > > +            * This page is now managed by the hugetlb allocator and has
> > > +            * no users -- drop the last reference.
> > > +            */
> > > +           zeroed = put_page_testzero(page);
> > > +           VM_BUG_ON_PAGE(!zeroed, page);
>
> Can this actually happen? AFAIK, page landed in update_and_free_page should be
> zero refcounted, then we increase the reference, and I cannot see how the
> reference might have changed in the meantime.

I am not sure whether other modules get the page and then put the
page. I see gather_surplus_pages does the same thing. So I copied
from there. I try to look at the memory_failure routine.


CPU0:                           CPU1:
                                set_compound_page_dtor(HUGETLB_PAGE_DTOR);
memory_failure_hugetlb
  get_hwpoison_page
    __get_hwpoison_page
      get_page_unless_zero
                                put_page_testzero()

Maybe this can happen. But it is a very corner case. If we want to
deal with this. We can put_page_testzero() first and then
set_compound_page_dtor(HUGETLB_PAGE_DTOR).

>
> I am all for catching corner cases, but not sure how realistic this is.
> Moreover, if we __ever__ get there, things can get nasty.
>
> We basically will have an in-use page in the free hugetlb pool, so corruption
> will happen. At that point, a plain BUG_ON might be better.
>
> But as I said, I do not think we need that.
>
> I yet need to look further, but what I have seen so far looks good.
>
> --
> Oscar Salvador
> SUSE L3