Re: [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb pages
From: Oscar Salvador
Date: Fri Feb 26 2021 - 04:46:07 EST
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
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here. I can see only one scenario (are there more?)
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__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:
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
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> chances to succeed next time.
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>
>
> With regard to the allocation, we restrict it to the node the page belongs
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> to with __GFP_THISNODE, meaning we do not fallback on other node's zones.
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>
>
> Note that gigantic hugetlb pages are fenced off since there is a cyclic
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> dependency between them and alloc_contig_range.
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>
>
> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
>
>
Thanks this looks much better than the initial version. One nit below.
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Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
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> +bool isolate_or_dissolve_huge_page(struct page *page)
>
> +{
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> + struct hstate *h = NULL;
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> + struct page *head;
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> + bool ret = false;
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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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> + }
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> + spin_unlock(&hugetlb_lock);
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> +
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> + /*
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> + * The page might have been dissolved from under our feet.
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> + * If that is the case, return success as if we dissolved it ourselves.
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> + */
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> + if (!h)
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> + return true;
>
>
I am still fighting with this construct a bit. It is not really clear
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what the lock is protecting us from here. alloc_fresh_huge_page deals
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with all potential races and this looks like an optimistic check to save
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some work. But in fact the lock is really necessary for correctness
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because hstate might be completely bogus without the lock or us holding
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a reference on the poage. The following construct would be more
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explicit and compact. What do you think?
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>
struct hstate *h;
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>
/*
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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.
--
Oscar Salvador
SUSE L3