Re: [patch 03/12] mm: huge_memory: use GFP_TRANSHUGE when charging huge pages

[Michal Hocko]

On Mon 16-06-14 15:54:23, Johannes Weiner wrote:
> Transparent huge page charges prefer falling back to regular pages
> rather than spending a lot of time in direct reclaim.
> 
> Desired reclaim behavior is usually declared in the gfp mask, but THP
> charges use GFP_KERNEL and then rely on the fact that OOM is disabled
> for THP charges, and that OOM-disabled charges currently skip reclaim.

OOM-disabled charges do one round of reclaim currently.

> Needless to say, this is anything but obvious and quite error prone.
> 
> Convert THP charges to use GFP_TRANSHUGE instead, which implies
> __GFP_NORETRY, to indicate the low-latency requirement.

OK, this makes sense. It would be ideal if we could use the same gfp as
for allocation but that would be too much churn I guess because some
allocator use a allocation helper which deduces proper gfp flags without
giving them back to the caller.

Nevertheless, I would still prefer if 05/12 was moved before
this patch because this is strictly speaking a behavior change.
 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Anyway
Acked-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/huge_memory.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e60837dc785c..10cd7f2bf776 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -827,7 +827,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		count_vm_event(THP_FAULT_FALLBACK);
>  		return VM_FAULT_FALLBACK;
>  	}
> -	if (unlikely(mem_cgroup_charge_anon(page, mm, GFP_KERNEL))) {
> +	if (unlikely(mem_cgroup_charge_anon(page, mm, GFP_TRANSHUGE))) {
>  		put_page(page);
>  		count_vm_event(THP_FAULT_FALLBACK);
>  		return VM_FAULT_FALLBACK;
> @@ -1101,7 +1101,7 @@ alloc:
>  		goto out;
>  	}
>  
> -	if (unlikely(mem_cgroup_charge_anon(new_page, mm, GFP_KERNEL))) {
> +	if (unlikely(mem_cgroup_charge_anon(new_page, mm, GFP_TRANSHUGE))) {
>  		put_page(new_page);
>  		if (page) {
>  			split_huge_page(page);
> @@ -2368,7 +2368,7 @@ static void collapse_huge_page(struct mm_struct *mm,
>  	if (!new_page)
>  		return;
>  
> -	if (unlikely(mem_cgroup_charge_anon(new_page, mm, GFP_KERNEL)))
> +	if (unlikely(mem_cgroup_charge_anon(new_page, mm, GFP_TRANSHUGE)))
>  		return;
>  
>  	/*
> -- 
> 2.0.0
> 

-- 
Michal Hocko
SUSE Labs
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