Re: [patch] mm, thp: do not allow thp faults to avoid cpuset restrictions

[Michal Hocko]

On Wed 23-07-14 15:50:09, David Rientjes wrote:
> The page allocator relies on __GFP_WAIT to determine if ALLOC_CPUSET 
> should be set in allocflags.  ALLOC_CPUSET controls if a page allocation 
> should be restricted only to the set of allowed cpuset mems.
> 
> Transparent hugepages clears __GFP_WAIT when defrag is disabled to prevent 
> the fault path from using memory compaction or direct reclaim.  Thus, it 
> is unfairly able to allocate outside of its cpuset mems restriction as a 
> side-effect.
> 
> This patch ensures that ALLOC_CPUSET is only cleared when the gfp mask is 
> truly GFP_ATOMIC by verifying it is also not a thp allocation.
> 
> Reported-by: Alex Thorlton <athorlton@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>

This is an abuse of __GFP_NO_KSWAPD but it also looks like a new gfp
flag would need to be added to do it in other way. No other users seem to
clear GFP_WAIT while using __GFP_NO_KSWAPD AFAICS so this should really
affect only THP allocations.

Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/page_alloc.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2447,7 +2447,7 @@ static inline int
>  gfp_to_alloc_flags(gfp_t gfp_mask)
>  {
>  	int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
> -	const gfp_t wait = gfp_mask & __GFP_WAIT;
> +	const bool atomic = !(gfp_mask & (__GFP_WAIT | __GFP_NO_KSWAPD));
>  
>  	/* __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch. */
>  	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
> @@ -2456,20 +2456,20 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	 * The caller may dip into page reserves a bit more if the caller
>  	 * cannot run direct reclaim, or if the caller has realtime scheduling
>  	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
> -	 * set both ALLOC_HARDER (!wait) and ALLOC_HIGH (__GFP_HIGH).
> +	 * set both ALLOC_HARDER (atomic == true) and ALLOC_HIGH (__GFP_HIGH).
>  	 */
>  	alloc_flags |= (__force int) (gfp_mask & __GFP_HIGH);
>  
> -	if (!wait) {
> +	if (atomic) {
>  		/*
> -		 * Not worth trying to allocate harder for
> -		 * __GFP_NOMEMALLOC even if it can't schedule.
> +		 * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
> +		 * if it can't schedule.
>  		 */
> -		if  (!(gfp_mask & __GFP_NOMEMALLOC))
> +		if (!(gfp_mask & __GFP_NOMEMALLOC))
>  			alloc_flags |= ALLOC_HARDER;
>  		/*
> -		 * Ignore cpuset if GFP_ATOMIC (!wait) rather than fail alloc.
> -		 * See also cpuset_zone_allowed() comment in kernel/cpuset.c.
> +		 * Ignore cpuset mems for GFP_ATOMIC rather than fail, see the
> +		 * comment for __cpuset_node_allowed_softwall().
>  		 */
>  		alloc_flags &= ~ALLOC_CPUSET;
>  	} else if (unlikely(rt_task(current)) && !in_interrupt())
> 
> --
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-- 
Michal Hocko
SUSE Labs
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