Re: [PATCH 1/1] mm/page_alloc: add a warning about high order allocations

[Michal Hocko]

Appart from general comments as a reply to the cover (btw. this all
should be in the changelog because this is the _why_ part of the
justification which should be _always_ part of the changelog).

On Tue 25-12-18 18:39:27, Konstantin Khorenko wrote:
[...]
> +config WARN_HIGH_ORDER
> +	bool "Enable complains about high order memory allocations"
> +	depends on !LOCKDEP

Why?

> +	default n
> +	help
> +	  Enables warnings on high order memory allocations. This allows to
> +	  determine users of large memory chunks and rework them to decrease
> +	  allocation latency. Note, some debug options make kernel structures
> +	  fat.
> +
> +config WARN_HIGH_ORDER_LEVEL
> +	int "Define page order level considered as too high"
> +	depends on WARN_HIGH_ORDER
> +	default 3
> +	help
> +	  Defines page order starting which the system to complain about.
> +	  Default is current PAGE_ALLOC_COSTLY_ORDER.
> +
>  config HWPOISON_INJECT
>  	tristate "HWPoison pages injector"
>  	depends on MEMORY_FAILURE && DEBUG_KERNEL && PROC_FS
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e95b5b7c9c3d..258892adb861 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4341,6 +4341,30 @@ static inline void finalise_ac(gfp_t gfp_mask, struct alloc_context *ac)
>  					ac->high_zoneidx, ac->nodemask);
>  }
>  
> +#ifdef CONFIG_WARN_HIGH_ORDER
> +int warn_order = CONFIG_WARN_HIGH_ORDER_LEVEL;
> +
> +/*
> + * Complain if we allocate a high order page unless there is a __GFP_NOWARN
> + * flag provided.
> + *
> + * Shuts up after 32 complains.
> + */
> +static __always_inline void warn_high_order(int order, gfp_t gfp_mask)
> +{
> +	static atomic_t warn_count = ATOMIC_INIT(32);
> +
> +	if (order >= warn_order && !(gfp_mask & __GFP_NOWARN))
> +		WARN(atomic_dec_if_positive(&warn_count) >= 0,
> +		     "order %d >= %d, gfp 0x%x\n",
> +		     order, warn_order, gfp_mask);
> +}

We do have ratelimit functionality, so why cannot you use it?

> +#else
> +static __always_inline void warn_high_order(int order, gfp_t gfp_mask)
> +{
> +}
> +#endif
> +
>  /*
>   * This is the 'heart' of the zoned buddy allocator.
>   */
> @@ -4361,6 +4385,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
>  		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
>  		return NULL;
>  	}
> +	warn_high_order(order, gfp_mask);
>  
>  	gfp_mask &= gfp_allowed_mask;
>  	alloc_mask = gfp_mask;

Why do you warn about all allocations in the hot path? I thought you
want to catch expensive allocations so I would assume that you would
stick that into a slow path after we are not able to allocate anything
after the first round of compaction.

Also do you want to warn about opportunistic GFP_NOWAIT allocations that
have a reasonable fallback?
-- 
Michal Hocko
SUSE Labs