Re: [PATCH v2 4/5] mm: Fix page reference leak in soft_offline_page()

[Oscar Salvador <osalvador@suse.de>]

On Tue, Jan 12, 2021 at 01:34:58AM -0800, Dan Williams wrote:
> The conversion to move pfn_to_online_page() internal to
> soft_offline_page() missed that the get_user_pages() reference needs to
> be dropped when pfn_to_online_page() fails.

I would be more specific here wrt. get_user_pages (madvise).
soft_offline_page gets called from more places besides madvise_*.

> When soft_offline_page() is handed a pfn_valid() &&
> !pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
> a leaked reference.
> 
> Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

LGTM, thanks for catching this:

Reviewed-by: Oscar Salvador <osalvador@suse.de>

A nit below.

> ---
>  mm/memory-failure.c |   20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5a38e9eade94..78b173c7190c 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1885,6 +1885,12 @@ static int soft_offline_free_page(struct page *page)
>  	return rc;
>  }
>  
> +static void put_ref_page(struct page *page)
> +{
> +	if (page)
> +		put_page(page);
> +}

I am not sure this warrants a function.
I would probably go with "if (ref_page).." in the two corresponding places,
but not feeling strong here.

> +
>  /**
>   * soft_offline_page - Soft offline a page.
>   * @pfn: pfn to soft-offline
> @@ -1910,20 +1916,26 @@ static int soft_offline_free_page(struct page *page)
>  int soft_offline_page(unsigned long pfn, int flags)
>  {
>  	int ret;
> -	struct page *page;
>  	bool try_again = true;
> +	struct page *page, *ref_page = NULL;
> +
> +	WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));

Did you see any scenario where this could happen? I understand that you are
adding this because we will leak a reference in case pfn is not valid anymore.

-- 
Oscar Salvador
SUSE L3
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