Re: [patch 10/12] mm: page_alloc: emergency reserve access for __GFP_NOFAIL allocations

[Michal Hocko]

On Wed 25-03-15 02:17:14, Johannes Weiner wrote:
> __GFP_NOFAIL allocations can deadlock the OOM killer when they're
> holding locks that the OOM victim might need to exit.  When that
> happens the allocation may never complete, which has disastrous
> effects on things like in-flight filesystem transactions.
> 
> When the system is OOM, allow __GFP_NOFAIL allocations to dip into the
> emergency reserves in the hope that this will allow transactions and
> writeback to complete and the deadlock can be avoided.

This one slipped through. Sorry.

> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/page_alloc.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c165016175d..832ad1c7cd4f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2403,9 +2403,17 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  	 * from exiting.  While allocations can use OOM kills to free
>  	 * memory, they can not necessarily rely on their *own* kills
>  	 * to make forward progress.
> +	 *
> +	 * This last point is crucial for __GFP_NOFAIL allocations.
> +	 * Since they can't quit, they might actually deadlock, so
> +	 * give them hail mary access to the emergency reserves.
>  	 */
> -	alloc_flags &= ~ALLOC_WMARK_MASK;
> -	alloc_flags |= ALLOC_WMARK_OOM;
> +	if (gfp_mask & __GFP_NOFAIL) {
> +		alloc_flags |= ALLOC_NO_WATERMARKS;
> +	} else {
> +		alloc_flags &= ~ALLOC_WMARK_MASK;
> +		alloc_flags |= ALLOC_WMARK_OOM;
> +	}
>  out:
>  	mutex_unlock(&oom_lock);
>  alloc:
> -- 
> 2.3.3
> 

-- 
Michal Hocko
SUSE Labs
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