Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

[Mike Kravetz]

On 4/13/21 3:47 AM, Oscar Salvador wrote:
> alloc_contig_range will fail if it ever sees a HugeTLB page within the
> range we are trying to allocate, even when that page is free and can be
> easily reallocated.
> This has proved to be problematic for some users of alloc_contic_range,
> e.g: CMA and virtio-mem, where those would fail the call even when those
> pages lay in ZONE_MOVABLE and are free.
> 
> We can do better by trying to replace such page.
> 
> Free hugepages are tricky to handle so as to no userspace application
> notices disruption, we need to replace the current free hugepage with
> a new one.
> 
> In order to do that, a new function called alloc_and_dissolve_huge_page
> is introduced.
> This function will first try to get a new fresh hugepage, and if it
> succeeds, it will replace the old one in the free hugepage pool.
> 
> The free page replacement is done under hugetlb_lock, so no external
> users of hugetlb will notice the change.
> To allocate the new huge page, we use alloc_buddy_huge_page(), so we
> do not have to deal with any counters, and prep_new_huge_page() is not
> called. This is valulable because in case we need to free the new page,
> we only need to call __free_pages().
> 
> Once we know that the page to be replaced is a genuine 0-refcounted
> huge page, we remove the old page from the freelist by remove_hugetlb_page().
> Then, we can call __prep_new_huge_page() and __prep_account_new_huge_page()
> for the new huge page to properly initialize it and increment the
> hstate->nr_huge_pages counter (previously decremented by
> remove_hugetlb_page()).
> Once done, the page is enqueued by enqueue_huge_page() and it is ready
> to be used.
> 
> There is one tricky case when
> page's refcount is 0 because it is in the process of being released.
> A missing PageHugeFreed bit will tell us that freeing is in flight so
> we retry after dropping the hugetlb_lock. The race window should be
> small and the next retry should make a forward progress.
> 
> E.g:
> 
> CPU0				CPU1
> free_huge_page()		isolate_or_dissolve_huge_page
> 				  PageHuge() == T
> 				  alloc_and_dissolve_huge_page
> 				    alloc_buddy_huge_page()
> 				    spin_lock_irq(hugetlb_lock)
> 				    // PageHuge() && !PageHugeFreed &&
> 				    // !PageCount()
> 				    spin_unlock_irq(hugetlb_lock)
>   spin_lock_irq(hugetlb_lock)
>   1) update_and_free_page
>        PageHuge() == F
>        __free_pages()
>   2) enqueue_huge_page
>        SetPageHugeFreed()
>   spin_unlock(&hugetlb_lock)

Very small nit, the above should be spin_unlock_irq(&hugetlb_lock)

> 				  spin_lock_irq(hugetlb_lock)
>                                    1) PageHuge() == F (freed by case#1 from CPU0)
> 				   2) PageHuge() == T
>                                        PageHugeFreed() == T
>                                        - proceed with replacing the page
> 
> In the case above 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@xxxxxxx>
> ---
...
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 0607b2b71ac6..4a664d6e82c1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2266,6 +2266,121 @@ static void restore_reserve_on_error(struct hstate *h,
>  	}
>  }
>  
> +/*
> + * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
> + * @h: struct hstate old page belongs to
> + * @old_page: Old page to dissolve
> + * Returns 0 on success, otherwise negated error.
> + */
> +
> +static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
> +{
> +	gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
> +	int nid = page_to_nid(old_page);
> +	struct page *new_page;
> +	int ret = 0;
> +
> +	/*
> +	 * Before dissolving the page, we need to allocate a new one for the
> +	 * pool to remain stable. Using alloc_buddy_huge_page() allows us to
> +	 * not having to deal with prep_new_page() and avoids dealing of any
> +	 * counters. This simplifies and let us do the whole thing under the
> +	 * lock.
> +	 */
> +	new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
> +	if (!new_page)
> +		return -ENOMEM;
> +
> +retry:
> +	spin_lock_irq(&hugetlb_lock);
> +	if (!PageHuge(old_page)) {
> +		/*
> +		 * Freed from under us. Drop new_page too.
> +		 */
> +		goto free_new;
> +	} else if (page_count(old_page)) {
> +		/*
> +		 * Someone has grabbed the page, fail for now.
> +		 */
> +		ret = -EBUSY;
> +		goto free_new;
> +	} else if (!HPageFreed(old_page)) {
> +		/*
> +		 * Page's refcount is 0 but it has not been enqueued in the
> +		 * freelist yet. Race window is small, so we can succeed here if
> +		 * we retry.
> +		 */
> +		spin_unlock_irq(&hugetlb_lock);
> +		cond_resched();
> +		goto retry;
> +	} else {
> +		/*
> +		 * Ok, old_page is still a genuine free hugepage. Remove it from
> +		 * the freelist and decrease the counters. These will be
> +		 * incremented again when calling __prep_account_new_huge_page()
> +		 * and enqueue_huge_page() for new_page. The counters will remain
> +		 * stable since this happens under the lock.
> +		 */
> +		remove_hugetlb_page(h, old_page, false);
> +
> +		/*
> +		 * Call __prep_new_huge_page() to construct the hugetlb page, and
> +		 * enqueue it then to place it in the freelists. After this,
> +		 * counters are back on track. Free hugepages have a refcount of 0,
> +		 * so we need to decrease new_page's count as well.
> +		 */
> +		__prep_new_huge_page(new_page);
> +		__prep_account_new_huge_page(h, nid);
> +		page_ref_dec(new_page);
> +		enqueue_huge_page(h, new_page);
> +
> +		/*
> +		 * Pages have been replaced, we can safely free the old one.
> +		 */
> +		spin_unlock_irq(&hugetlb_lock);
> +		update_and_free_page(h, old_page);
> +	}
> +
> +	return ret;
> +
> +free_new:
> +	spin_unlock_irq(&hugetlb_lock);
> +	__free_pages(new_page, huge_page_order(h));
> +
> +	return ret;
> +}
> +
> +int isolate_or_dissolve_huge_page(struct page *page)
> +{
> +	struct hstate *h;
> +	struct page *head;
> +
> +	/*
> +	 * The page might have been dissolved from under our feet, so make sure
> +	 * to carefully check the state under the lock.
> +	 * Return success when racing as if we dissolved the page ourselves.
> +	 */
> +	spin_lock_irq(&hugetlb_lock);
> +	if (PageHuge(page)) {
> +		head = compound_head(page);
> +		h = page_hstate(head);
> +	} else {
> +		spin_unlock(&hugetlb_lock);

Should be be spin_unlock_irq(&hugetlb_lock);

Other than that, it looks good.
-- 
Mike Kravetz