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

[Vlastimil Babka <vbabka@suse.cz>]

On 10/13/2017 02:07 PM, Michal Hocko wrote:
> On Fri 13-10-17 14:04:08, Vlastimil Babka wrote:
>> On 10/13/2017 02:00 PM, Michal Hocko wrote:
>>> From: Michal Hocko <mhocko@suse.com>
>>>
>>> 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@suse.com>
>>> ---
>>>  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"
> 
> Dohh, you are right of course. I blame auto-completion ;) but I am lame
> in fact...
> ---
> From 44b20bdb03846bc5fd79c883d16b8f3aa436878f Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Fri, 13 Oct 2017 13:55:21 +0200
> Subject: [PATCH] mm, page_alloc: fail has_unmovable_pages when seeing reserved
>  pages
> 
> 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@suse.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ad0294ab3e4f..5b4d85ae445c 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 (PageReserved(page))
> +			return true;
> +
>  		/*
>  		 * Hugepages are not in LRU lists, but they're movable.
>  		 * We need not scan over tail pages bacause we don't
>