Re: [PATCH 2/4] memcg: unify force charging conditions

[Roman Gushchin <guro@fb.com>]

On Thu, Feb 10, 2022 at 12:14:35AM -0800, Shakeel Butt wrote:
> Currently the kernel force charges the allocations which have __GFP_HIGH
> flag without triggering the memory reclaim. __GFP_HIGH indicates that
> the caller is high priority and since commit 869712fd3de5 ("mm:
> memcontrol: fix network errors from failing __GFP_ATOMIC charges") the
> kernel let such allocations do force charging. Please note that
> __GFP_ATOMIC has been replaced by __GFP_HIGH.
> 
> __GFP_HIGH does not tell if the caller can block or can trigger reclaim.
> There are separate checks to determine that. So, there is no need to
> skip reclaim for __GFP_HIGH allocations. So, handle __GFP_HIGH together
> with __GFP_NOFAIL which also does force charging.

This sounds very reasonable. But shouldn't we check if __GFP_DIRECT_RECLAIM
is set and bail out otherwise?

Thanks!

> 
> Please note that this is a noop change as there are no __GFP_HIGH
> allocators in kernel which also have __GFP_ACCOUNT (or SLAB_ACCOUNT) and
> does not allow reclaim for now. The reason for this patch is to simplify
> the reasoning of the following patches.
> 
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> ---
>  mm/memcontrol.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c40c27822802..ae73a40818b0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2560,15 +2560,6 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  		goto retry;
>  	}
>  
> -	/*
> -	 * Memcg doesn't have a dedicated reserve for atomic
> -	 * allocations. But like the global atomic pool, we need to
> -	 * put the burden of reclaim on regular allocation requests
> -	 * and let these go through as privileged allocations.
> -	 */
> -	if (gfp_mask & __GFP_HIGH)
> -		goto force;
> -
>  	/*
>  	 * Prevent unbounded recursion when reclaim operations need to
>  	 * allocate memory. This might exceed the limits temporarily,
> @@ -2642,7 +2633,13 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  		goto retry;
>  	}
>  nomem:
> -	if (!(gfp_mask & __GFP_NOFAIL))
> +	/*
> +	 * Memcg doesn't have a dedicated reserve for atomic
> +	 * allocations. But like the global atomic pool, we need to
> +	 * put the burden of reclaim on regular allocation requests
> +	 * and let these go through as privileged allocations.
> +	 */
> +	if (!(gfp_mask & (__GFP_NOFAIL | __GFP_HIGH)))
>  		return -ENOMEM;
>  force:
>  	/*
> -- 
> 2.35.1.265.g69c8d7142f-goog
>