Re: [PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without triggering reclaim

[Vlastimil Babka]

On 11/16/2015 02:22 PM, mhocko@xxxxxxxxxx wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> __alloc_pages_slowpath is looping over ALLOC_NO_WATERMARKS requests if
> __GFP_NOFAIL is requested. This is fragile because we are basically
> relying on somebody else to make the reclaim (be it the direct reclaim
> or OOM killer) for us. The caller might be holding resources (e.g.
> locks) which block other other reclaimers from making any progress for
> example. Remove the retry loop and rely on __alloc_pages_slowpath to
> invoke all allowed reclaim steps and retry logic.
> 
> We have to be careful about __GFP_NOFAIL allocations from the
> PF_MEMALLOC context even though this is a very bad idea to begin with
> because no progress can be gurateed at all.  We shouldn't break the
> __GFP_NOFAIL semantic here though. It could be argued that this is
> essentially GFP_NOWAIT context which we do not support but PF_MEMALLOC
> is much harder to check for existing users because they might happen
> deep down the code path performed much later after setting the flag
> so we cannot really rule out there is no kernel path triggering this
> combination.
> 
> Acked-by: Mel Gorman <mgorman@xxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/page_alloc.c | 32 ++++++++++++++++++--------------
>  1 file changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b153fa3d0b9b..df7746280427 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3046,32 +3046,36 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		 * allocations are system rather than user orientated
>  		 */
>  		ac->zonelist = node_zonelist(numa_node_id(), gfp_mask);
> -		do {
> -			page = get_page_from_freelist(gfp_mask, order,
> -							ALLOC_NO_WATERMARKS, ac);
> -			if (page)
> -				goto got_pg;
> -
> -			if (gfp_mask & __GFP_NOFAIL)
> -				wait_iff_congested(ac->preferred_zone,
> -						   BLK_RW_ASYNC, HZ/50);

I've been thinking if the lack of unconditional wait_iff_congested() can affect
something negatively. I guess not?

> -		} while (gfp_mask & __GFP_NOFAIL);
> +		page = get_page_from_freelist(gfp_mask, order,
> +						ALLOC_NO_WATERMARKS, ac);
> +		if (page)
> +			goto got_pg;
>  	}
>  
>  	/* Caller is not willing to reclaim, we can't balance anything */
>  	if (!can_direct_reclaim) {
>  		/*
> -		 * All existing users of the deprecated __GFP_NOFAIL are
> -		 * blockable, so warn of any new users that actually allow this
> -		 * type of allocation to fail.
> +		 * All existing users of the __GFP_NOFAIL are blockable, so warn
> +		 * of any new users that actually allow this type of allocation
> +		 * to fail.
>  		 */
>  		WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
>  		goto nopage;
>  	}
>  
>  	/* Avoid recursion of direct reclaim */
> -	if (current->flags & PF_MEMALLOC)
> +	if (current->flags & PF_MEMALLOC) {
> +		/*
> +		 * __GFP_NOFAIL request from this context is rather bizarre
> +		 * because we cannot reclaim anything and only can loop waiting
> +		 * for somebody to do a work for us.
> +		 */
> +		if (WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> +			cond_resched();
> +			goto retry;
> +		}
>  		goto nopage;
> +	}
>  
>  	/* Avoid allocations with no watermarks from looping endlessly */
>  	if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
> 

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