Re: [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb pages

Oscar Salvador
2021-02-26T09:46:23Z

On Fri, Feb 26, 2021 at 09:35:09AM +0100, Michal Hocko wrote:
> I think it would be helpful to call out that specific case explicitly
> here. I can see only one scenario (are there more?)
> __free_huge_page() isolate_or_dissolve_huge_page
> PageHuge() == T
> alloc_and_dissolve_huge_page
> alloc_fresh_huge_page()
> spin_lock(hugetlb_lock)
> // PageHuge() && !PageHugeFreed &&
> // !PageCount()
> spin_unlock(hugetlb_lock)
> spin_lock(hugetlb_lock)
> 1) update_and_free_page
> PageHuge() == F
> __free_pages()
> 2) enqueue_huge_page
> SetPageHugeFreed()
> spin_unlock(&hugetlb_lock)

I do not think there are more scenarios. The thing is to find a !page_count &&
!PageHugeFreed.
AFAICS, this can only happen after:
put_page->put_page_test_zero->..->_free_huge_page gets called but __free_huge_page
has not reached enqueue_huge_page yet (as you just described above)

By calling out this case, you meant to describe it in the changelog?

>
> > In this case we retry as the window race is quite small and we have high
> > chances to succeed next time.
> >
> > With regard to the allocation, we restrict it to the node the page belongs
> > to with __GFP_THISNODE, meaning we do not fallback on other node's zones.
> >
> > Note that gigantic hugetlb pages are fenced off since there is a cyclic
> > dependency between them and alloc_contig_range.
> >
> > Signed-off-by: Oscar Salvador <osalvador@suse.de>
>
> Thanks this looks much better than the initial version. One nit below.
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> > +bool isolate_or_dissolve_huge_page(struct page *page)
> > +{
> > + struct hstate *h = NULL;
> > + struct page *head;
> > + bool ret = false;
> > +
> > + spin_lock(&hugetlb_lock);
> > + if (PageHuge(page)) {
> > + head = compound_head(page);
> > + h = page_hstate(head);
> > + }
> > + spin_unlock(&hugetlb_lock);
> > +
> > + /*
> > + * The page might have been dissolved from under our feet.
> > + * If that is the case, return success as if we dissolved it ourselves.
> > + */
> > + if (!h)
> > + return true;
>
> I am still fighting with this construct a bit. It is not really clear
> what the lock is protecting us from here. alloc_fresh_huge_page deals
> with all potential races and this looks like an optimistic check to save
> some work. But in fact the lock is really necessary for correctness
> because hstate might be completely bogus without the lock or us holding
> a reference on the poage. The following construct would be more
> explicit and compact. What do you think?
>
> struct hstate *h;
>
> /*
> * The page might have been dissloved from under our feet
> * so make sure to carefully check the state under the lock.
> * Return success on when racing as if we dissloved the page
> * ourselves.
> */
> spin_lock(&hugetlb_lock);
> if (PageHuge(page)) {
> head = compound_head(page);
> h = page_hstate(head);
> } else {
> spin_unlock(&hugetlb_lock);
> return true;
> }
> spin_unlock(&hugetlb_lock);

Yes, I find this less eyesore.

I will fix it up in v4.

--
Oscar Salvador
SUSE L3