Re: [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb pages
From: Michal Hocko
Date: Fri Feb 26 2021 - 04:52:30 EST
On Fri 26-02-21 10:45:14, Oscar Salvador wrote:
>
On Fri, Feb 26, 2021 at 09:35:09AM +0100, Michal Hocko wrote:
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> 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
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> PageHuge() == T
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> alloc_and_dissolve_huge_page
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> alloc_fresh_huge_page()
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> spin_lock(hugetlb_lock)
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> // PageHuge() && !PageHugeFreed &&
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> // !PageCount()
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> spin_unlock(hugetlb_lock)
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> spin_lock(hugetlb_lock)
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> 1) update_and_free_page
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> PageHuge() == F
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> __free_pages()
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> 2) enqueue_huge_page
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> SetPageHugeFreed()
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> 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:
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put_page->put_page_test_zero->..->_free_huge_page gets called but __free_huge_page
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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.
[...]
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> struct hstate *h;
>
>
>
> /*
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> * The page might have been dissloved from under our feet
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> * so make sure to carefully check the state under the lock.
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> * Return success on when racing as if we dissloved the page
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> * ourselves.
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> */
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> spin_lock(&hugetlb_lock);
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> if (PageHuge(page)) {
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> head = compound_head(page);
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> h = page_hstate(head);
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> } else {
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> spin_unlock(&hugetlb_lock);
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> return true;
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> }
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> spin_unlock(&hugetlb_lock);
>
>
Yes, I find this less eyesore.
>
>
I will fix it up in v4.
Thanks!
--
Michal Hocko
SUSE Labs