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

Michal Hocko
2021-02-26T09:52:54Z

On Fri 26-02-21 10:45:14, Oscar Salvador wrote:
> 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?

Yes.
[...]
> > 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.

Thanks!
--
Michal Hocko
SUSE Labs