Mel Gorman wrote:
> out_of_memory() may be called when an allocation is failing and the direct
> reclaim is not making any progress. This does not take into account the
> requested order of the allocation. If the request if for an order larger
> than PAGE_ALLOC_COSTLY_ORDER, it is reasonable to fail the allocation
> because the kernel makes no guarantees about those allocations succeeding.
>
> This false OOM situation can occur if a user is trying to grow the hugepage
> pool in a script like;
>
> #!/bin/bash
> REQUIRED=$1
> echo 1 > /proc/sys/vm/hugepages_treat_as_movable
> echo $REQUIRED > /proc/sys/vm/nr_hugepages
> ACTUAL=`cat /proc/sys/vm/nr_hugepages`
> while [ $REQUIRED -ne $ACTUAL ]; do
> echo Huge page pool at $ACTUAL growing to $REQUIRED
> echo $REQUIRED > /proc/sys/vm/nr_hugepages
> ACTUAL=`cat /proc/sys/vm/nr_hugepages`
> sleep 1
> done
>
> This is a reasonable scenario when ZONE_MOVABLE is in use but triggers OOM
> easily on 2.6.23-rc1. This patch will fail an allocation for an order above
> PAGE_ALLOC_COSTLY_ORDER instead of killing processes and retrying.
>
> Signed-off-by: Mel Gorman <[email protected]>
We have had this problem for a long time. When allocating large pages
we could find ourselves unable to allocate such a page nor reclaim one
for ourselves. At this point we will OOM with little hope of that
actually changing the situation for the better.
As you say PAGE_ALLOC_COSTLY_ORDER pretty much defines the orders at
which any sort of guarantee of success is provided. It seems preferable
to fail a allocations above this order/ than killing things to try and
make it available. As higher order users already have to handle failure
to allocate they should be best equipped to continue.
Acked-by: Andy Whitcroft <[email protected]>
> ---
> page_alloc.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 40954fb..da57173 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1350,6 +1350,10 @@ nofail_alloc:
> if (page)
> goto got_pg;
>
> + /* The OOM killer will not help higher order allocs so fail */
> + if (order > PAGE_ALLOC_COSTLY_ORDER)
> + goto nopage;
> +
> out_of_memory(zonelist, gfp_mask, order);
> goto restart;
> }
-apw
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