Re: [PATCH v4 05/11] mm/migrate: clear __GFP_RECLAIM for THP allocation for migration

[Vlastimil Babka]

On 7/7/20 9:44 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> In mm/migrate.c, THP allocation for migration is called with the provided
> gfp_mask | GFP_TRANSHUGE. This gfp_mask contains __GFP_RECLAIM and it
> would be conflict with the intention of the GFP_TRANSHUGE.
> 
> GFP_TRANSHUGE/GFP_TRANSHUGE_LIGHT is introduced to control the reclaim
> behaviour by well defined manner since overhead of THP allocation is
> quite large and the whole system could suffer from it. So, they deals
> with __GFP_RECLAIM mask deliberately. If gfp_mask contains __GFP_RECLAIM
> and uses gfp_mask | GFP_TRANSHUGE(_LIGHT) for THP allocation, it means
> that it breaks the purpose of the GFP_TRANSHUGE(_LIGHT).
> 
> This patch fixes this situation by clearing __GFP_RECLAIM in provided
> gfp_mask. Note that there are some other THP allocations for migration
> and they just uses GFP_TRANSHUGE(_LIGHT) directly. This patch would make
> all THP allocation for migration consistent.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  mm/migrate.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 02b31fe..ecd7615 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1547,6 +1547,11 @@ struct page *new_page_nodemask(struct page *page,
>  	}
>  
>  	if (PageTransHuge(page)) {
> +		/*
> +		 * clear __GFP_RECALIM since GFP_TRANSHUGE is the gfp_mask
> +		 * that chooses the reclaim masks deliberately.
> +		 */
> +		gfp_mask &= ~__GFP_RECLAIM;
>  		gfp_mask |= GFP_TRANSHUGE;

In addition to what Michal said...

The mask is not passed to this function, so I would just redefine it, as is done
in the hugetlb case. We probably don't even need the __GFP_RETRY_MAYFAIL for the
THP case asi it's just there to prevent OOM kill (per commit 0f55685627d6d ) and
the costly order of THP is enough for that.

>  		order = HPAGE_PMD_ORDER;
>  	}
>