linux-kernel - Re: [PATCH v3 1/2] mm: Make alloc_contig

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Message-ID: <YDjEnKex9bBmQv7C@dhcp22.suse.cz>
Date:   Fri, 26 Feb 2021 10:51:24 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb
 pages

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