[PATCH] mm: remove __GFP_NOFAIL is deprecated comment

[PATCH] mm: remove __GFP_NOFAIL is deprecated comment

[Michal Hocko]

From: Michal Hocko <mhocko@suse.com>

647757197cd3 ("mm: clarify __GFP_NOFAIL deprecation status") was
incomplete and didn't remove the comment about __GFP_NOFAIL being
deprecated in buffered_rmqueue. Let's get rid of this leftover
but keep the WARN_ON_ONCE for order > 1 because we should really
discourage from using __GFP_NOFAIL with higher order allocations
because those are just too subtle.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
Hi,
this popped out when discussing another patch http://lkml.kernel.org/r/56CEC568.6080809@kyup.com
so I think it is worth removing the comment.

 mm/page_alloc.c | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1993894b4219..109d975a7172 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2347,19 +2347,11 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
 		list_del(&page->lru);
 		pcp->count--;
 	} else {
-		if (unlikely(gfp_flags & __GFP_NOFAIL)) {
-			/*
-			 * __GFP_NOFAIL is not to be used in new code.
-			 *
-			 * All __GFP_NOFAIL callers should be fixed so that they
-			 * properly detect and handle allocation failures.
-			 *
-			 * We most definitely don't want callers attempting to
-			 * allocate greater than order-1 page units with
-			 * __GFP_NOFAIL.
-			 */
-			WARN_ON_ONCE(order > 1);
-		}
+		/*
+		 * We most definitely don't want callers attempting to
+		 * allocate greater than order-1 page units with __GFP_NOFAIL.
+		 */
+		WARN_ON_ONCE(unlikely(gfp_flags & __GFP_NOFAIL) && (order > 1));
 		spin_lock_irqsave(&zone->lock, flags);
 
 		page = NULL;
-- 
2.7.0

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Re: [PATCH] mm: remove __GFP_NOFAIL is deprecated comment

[Nikolay Borisov]

On 02/25/2016 12:43 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> 647757197cd3 ("mm: clarify __GFP_NOFAIL deprecation status") was
> incomplete and didn't remove the comment about __GFP_NOFAIL being
> deprecated in buffered_rmqueue. Let's get rid of this leftover
> but keep the WARN_ON_ONCE for order > 1 because we should really
> discourage from using __GFP_NOFAIL with higher order allocations
> because those are just too subtle.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> Hi,
> this popped out when discussing another patch http://lkml.kernel.org/r/56CEC568.6080809@kyup.com
> so I think it is worth removing the comment.
> 
>  mm/page_alloc.c | 18 +++++-------------
>  1 file changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1993894b4219..109d975a7172 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2347,19 +2347,11 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
>  		list_del(&page->lru);
>  		pcp->count--;
>  	} else {
> -		if (unlikely(gfp_flags & __GFP_NOFAIL)) {
> -			/*
> -			 * __GFP_NOFAIL is not to be used in new code.
> -			 *
> -			 * All __GFP_NOFAIL callers should be fixed so that they
> -			 * properly detect and handle allocation failures.
> -			 *
> -			 * We most definitely don't want callers attempting to
> -			 * allocate greater than order-1 page units with
> -			 * __GFP_NOFAIL.
> -			 */
> -			WARN_ON_ONCE(order > 1);
> -		}
> +		/*
> +		 * We most definitely don't want callers attempting to
> +		 * allocate greater than order-1 page units with __GFP_NOFAIL.
> +		 */
> +		WARN_ON_ONCE(unlikely(gfp_flags & __GFP_NOFAIL) && (order > 1));

WARN_ON_ONCE already includes an unlikely in its definition:
http://lxr.free-electrons.com/source/include/asm-generic/bug.h#L109

>  		spin_lock_irqsave(&zone->lock, flags);
>  
>  		page = NULL;
> 


Reviewed-by: Nikolay Borisov <kernel@kyup.com>

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Re: [PATCH] mm: remove __GFP_NOFAIL is deprecated comment

[Michal Hocko]

On Thu 25-02-16 13:36:11, Nikolay Borisov wrote:
[...]
> > +		/*
> > +		 * We most definitely don't want callers attempting to
> > +		 * allocate greater than order-1 page units with __GFP_NOFAIL.
> > +		 */
> > +		WARN_ON_ONCE(unlikely(gfp_flags & __GFP_NOFAIL) && (order > 1));
> 
> WARN_ON_ONCE already includes an unlikely in its definition:
> http://lxr.free-electrons.com/source/include/asm-generic/bug.h#L109

OK, I just wanted to keep the condition untouched but you are right the
unlikely can be removed.

> Reviewed-by: Nikolay Borisov <kernel@kyup.com>

Thanks!
---

Re: [PATCH] mm: remove __GFP_NOFAIL is deprecated comment

[Vlastimil Babka]

On 02/25/2016 02:48 PM, Michal Hocko wrote:
> On Thu 25-02-16 13:36:11, Nikolay Borisov wrote:
> -		if (unlikely(gfp_flags & __GFP_NOFAIL)) {
> -			/*
> -			 * __GFP_NOFAIL is not to be used in new code.
> -			 *
> -			 * All __GFP_NOFAIL callers should be fixed so that they
> -			 * properly detect and handle allocation failures.
> -			 *
> -			 * We most definitely don't want callers attempting to
> -			 * allocate greater than order-1 page units with
> -			 * __GFP_NOFAIL.
> -			 */
> -			WARN_ON_ONCE(order > 1);
> -		}
> +		/*
> +		 * We most definitely don't want callers attempting to
> +		 * allocate greater than order-1 page units with __GFP_NOFAIL.
> +		 */
> +		WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
>   		spin_lock_irqsave(&zone->lock, flags);
>
>   		page = NULL;
>

Hmm, even the reduced text (and the WARN_ON in the first place) sounds 
IMHO discouraging enough to make people think that opencoding a loop 
around such allocations is a good workaround. Yeah, we have a 
better/more thorough explanation around the __GFP_NOFAIL definition, but 
the WARN_ON will point people here.

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