Re: [PATCH v2 1/2] mm: free compound page with correct order

[Andrew Morton]

On Wed, 15 Oct 2014 12:20:04 -0700 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:

> Compound page should be freed by put_page() or free_pages() with
> correct order. Not doing so will cause tail pages leaked.
> 
> The compound order can be obtained by compound_order() or use
> HPAGE_PMD_ORDER in our case. Some people would argue the latter
> is faster but I prefer the former which is more general.
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Fixes: 97ae17497e99 ("thp: implement refcounting for huge zero page")
> Cc: stable@xxxxxxxxxxxxxxx (v3.8+)

It's two years old and nobody noticed the memory leak, so presumably it
happens rarely.

> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 74c78aa..780d12c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -200,7 +200,7 @@ retry:
>  	preempt_disable();
>  	if (cmpxchg(&huge_zero_page, NULL, zero_page)) {
>  		preempt_enable();
> -		__free_page(zero_page);
> +		__free_pages(zero_page, compound_order(zero_page));

This is rare.

>  		goto retry;
>  	}
>  
> @@ -232,7 +232,7 @@ static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
>  	if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
>  		struct page *zero_page = xchg(&huge_zero_page, NULL);
>  		BUG_ON(zero_page == NULL);
> -		__free_page(zero_page);
> +		__free_pages(zero_page, compound_order(zero_page));

But I'm surprised that this is also rare.  It makes me wonder if this
code is working correctly.

>  		return HPAGE_PMD_NR;
>  	}

Were you able to observe the leakage in practice?  If so, under what
circumstances?

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