Re: [PATCH 2/2] mm: warn about ALLOC_NO_WATERMARKS request failures

[David Rientjes]

On Wed, 25 Nov 2015, Michal Hocko wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> ALLOC_NO_WATERMARKS requests can dive into memory reserves without any
> restriction. They are used only in the case of emergency to allow
> forward memory reclaim progress assuming the caller should return the
> memory in a short time (e.g. {__GFP,PF}_MEMALLOC requests or OOM victim
> on the way to exit or __GFP_NOFAIL requests hitting OOM). There is no
> guarantee such request succeed because memory reserves might get
> depleted as well. This might be either a result of a bug where memory
> reserves are abused or a result of a too optimistic configuration of
> memory reserves.
> 
> This patch makes sure that the administrator gets a warning when these
> requests fail with a hint that min_free_kbytes might be used to increase
> the amount of memory reserves. The warning might also help us check
> whether the issue is caused by a buggy user or the configuration. To
> prevent from flooding the logs the warning is on off but we allow it to
> trigger again after min_free_kbytes was updated. Something really bad is
> clearly going on if the warning hits even after multiple updates of
> min_free_kbytes.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/page_alloc.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 70db11c27046..6a05d771cb08 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -240,6 +240,8 @@ compound_page_dtor * const compound_page_dtors[] = {
>  #endif
>  };
>  
> +/* warn about depleted watermarks */
> +static bool warn_alloc_no_wmarks;
>  int min_free_kbytes = 1024;
>  int user_min_free_kbytes = -1;
>  
> @@ -2642,6 +2644,13 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  	if (zonelist_rescan)
>  		goto zonelist_scan;
>  
> +	/* WARN only once unless min_free_kbytes is updated */
> +	if (warn_alloc_no_wmarks && (alloc_flags & ALLOC_NO_WATERMARKS)) {
> +		warn_alloc_no_wmarks = 0;
> +		WARN(1, "Memory reserves are depleted for order:%d, mode:0x%x."
> +			" You might consider increasing min_free_kbytes\n",
> +			order, gfp_mask);
> +	}
>  	return NULL;
>  }
>  

Doesn't this warn for high-order allocations prior to the first call to 
direct compaction whereas min_free_kbytes may be irrelevant?  Providing 
the order is good, but there's no indication when min_free_kbytes may be 
helpful from this warning.  WARN() isn't even going to show the state of 
memory.
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