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

[Michal Hocko <mhocko@suse.com>]

On Tue 13-04-21 12:47:45, 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)
> 				  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@suse.de>

Acked-by: Michal Hocko <mhocko@suse.com>

One minor nit below
[...]

> +		/*
> +		 * 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);

I think it would help to put something like the following into the
comment above this really strange construct.

		/*
		 * new_page needs to be initialized with the standard
		 * hugetlb state. This is normally done by
		 * prep_new_huge_page but that takes hugetlb_lock which
		 * is already held so we need to open code it here.
		 * Reference count trick is needed because allocator
		 * gives us referenced page but the pool requires pages
		 * with 0 refcount.
		 */

> +		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);

-- 
Michal Hocko
SUSE Labs