Re: [patch 11/18] oom: avoid oom killer for lowmem allocations

[Andrew Morton <akpm@linux-foundation.org>]

> oom: avoid oom killer for lowmem allocations

I think the terminology is poor.  My 256MB test box only has lowmem! 
In the past we've used the term "lower zone" here, which is I think
what you want?

On Sun, 6 Jun 2010 15:34:38 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> If memory has been depleted in lowmem zones even with the protection
> afforded to it by /proc/sys/vm/lowmem_reserve_ratio, it is unlikely that
> killing current users will help.  The memory is either reclaimable (or
> migratable) already, in which case we should not invoke the oom killer at
> all, or it is pinned by an application for I/O.  Killing such an
> application may leave the hardware in an unspecified state and there is no
> guarantee that it will be able to make a timely exit.

Killing an application can leave hardware in an unspecified state?  How
so?  That means a ^C kills the box!

> Lowmem allocations are now failed in oom conditions when __GFP_NOFAIL is
> not used so that the task can perhaps recover or try again later.
> 
> Previously, the heuristic provided some protection for those tasks with
> CAP_SYS_RAWIO, but this is no longer necessary since we will not be
> killing tasks for the purposes of ISA allocations.
> 
> high_zoneidx is gfp_zone(gfp_flags), meaning that ZONE_NORMAL will be the
> default for all allocations that are not __GFP_DMA, __GFP_DMA32,
> __GFP_HIGHMEM, and __GFP_MOVABLE on kernels configured to support those
> flags.  Testing for high_zoneidx being less than ZONE_NORMAL will only
> return true for allocations that have either __GFP_DMA or __GFP_DMA32.
> 
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  mm/page_alloc.c |   29 ++++++++++++++++++++---------
>  1 files changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1759,6 +1759,9 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  		/* The OOM killer will not help higher order allocs */
>  		if (order > PAGE_ALLOC_COSTLY_ORDER)
>  			goto out;
> +		/* The OOM killer does not needlessly kill tasks for lowmem */

a) terminology is scary

b) comment doesn't explain _why_, which is the most important thing
   to explain.

> +		if (high_zoneidx < ZONE_NORMAL)
> +			goto out;
>  		/*
>  		 * GFP_THISNODE contains __GFP_NORETRY and we never hit this.
>  		 * Sanity check for bare calls of __GFP_THISNODE, not real OOM.
> @@ -2052,15 +2055,23 @@ rebalance:
>  			if (page)
>  				goto got_pg;
>  
> -			/*
> -			 * The OOM killer does not trigger for high-order
> -			 * ~__GFP_NOFAIL allocations so if no progress is being
> -			 * made, there are no other options and retrying is
> -			 * unlikely to help.
> -			 */
> -			if (order > PAGE_ALLOC_COSTLY_ORDER &&
> -						!(gfp_mask & __GFP_NOFAIL))
> -				goto nopage;
> +			if (!(gfp_mask & __GFP_NOFAIL)) {
> +				/*
> +				 * The oom killer is not called for high-order
> +				 * allocations that may fail, so if no progress
> +				 * is being made, there are no other options and
> +				 * retrying is unlikely to help.
> +				 */
> +				if (order > PAGE_ALLOC_COSTLY_ORDER)
> +					goto nopage;
> +				/*
> +				 * The oom killer is not called for lowmem
> +				 * allocations to prevent needlessly killing
> +				 * innocent tasks.
> +				 */

s/lowmem/somethingelse/

> +				if (high_zoneidx < ZONE_NORMAL)
> +					goto nopage;
> +			}
>  
>  			goto restart;
>  		}

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