Re: [PATCH 2/2] mm, page_alloc: fail has_unmovable_pages when seeing reserved pages

[Vlastimil Babka]

On 10/13/2017 02:00 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Reserved pages should be completely ignored by the core mm because they
> have a special meaning for their owners. has_unmovable_pages doesn't
> check those so we rely on other tests (reference count, or PageLRU) to
> fail on such pages. Althought this happens to work it is safer to simply
> check for those explicitly and do not rely on the owner of the page
> to abuse those fields for special purposes.
> 
> Please note that this is more of a further fortification of the code
> rahter than a fix of an existing issue.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/page_alloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ad0294ab3e4f..a8800b0a5619 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7365,6 +7365,9 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>  
>  		page = pfn_to_page(check);
>  
> +		if (PageReferenced(page))

"Referenced" != "Reserved"

> +			return true;
> +
>  		/*
>  		 * Hugepages are not in LRU lists, but they're movable.
>  		 * We need not scan over tail pages bacause we don't
>