Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

[Colin Cross]

On Tue, Oct 25, 2011 at 2:09 AM, Mel Gorman <mgorman@xxxxxxx> wrote:
> On Mon, Oct 24, 2011 at 11:39:49PM -0700, Colin Cross wrote:
>> Under the following conditions, __alloc_pages_slowpath can loop
>> forever:
>> gfp_mask & __GFP_WAIT is true
>> gfp_mask & __GFP_FS is false
>> reclaim and compaction make no progress
>> order <= PAGE_ALLOC_COSTLY_ORDER
>>
>> These conditions happen very often during suspend and resume,
>> when pm_restrict_gfp_mask() effectively converts all GFP_KERNEL
>> allocations into __GFP_WAIT.
> b>
>> The oom killer is not run because gfp_mask & __GFP_FS is false,
>> but should_alloc_retry will always return true when order is less
>> than PAGE_ALLOC_COSTLY_ORDER.
>>
>> Fix __alloc_pages_slowpath to skip retrying when oom killer is
>> not allowed by the GFP flags, the same way it would skip if the
>> oom killer was allowed but disabled.
>>
>> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
>
> Hi Colin,
>
> Your patch functionally seems fine. I see the problem and we certainly
> do not want to have the OOM killer firing during suspend. I would prefer
> that the IO devices would not be suspended until reclaim was completed
> but I imagine that would be a lot harder.
>
> That said, it will be difficult to remember why checking __GFP_NOFAIL in
> this case is necessary and someone might "optimitise" it away later. It
> would be preferable if it was self-documenting. Maybe something like
> this? (This is totally untested)

This issue is not limited to suspend, any GFP_NOIO allocation could
end up in the same loop.  Suspend is the most likely case, because it
effectively converts all GFP_KERNEL allocations into GFP_NOIO.

>  mm/page_alloc.c |   22 ++++++++++++++++++++++
>  1 files changed, 22 insertions(+), 0 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6e8ecb6..ad8f376 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -127,6 +127,20 @@ void pm_restrict_gfp_mask(void)
>        saved_gfp_mask = gfp_allowed_mask;
>        gfp_allowed_mask &= ~GFP_IOFS;
>  }
> +
> +static bool pm_suspending(void)
> +{
> +       if ((gfp_allowed_mask & GFP_IOFS) == GFP_IOFS)
> +               return false;
> +       return true;
> +}
> +
> +#else
> +
> +static bool pm_suspending(void)
> +{
> +       return false;
> +}
>  #endif /* CONFIG_PM_SLEEP */
>
>  #ifdef CONFIG_HUGETLB_PAGE_SIZE_VARIABLE
> @@ -2207,6 +2221,14 @@ rebalance:
>
>                        goto restart;
>                }
> +
> +               /*
> +                * Suspend converts GFP_KERNEL to __GFP_WAIT which can
> +                * prevent reclaim making forward progress without
> +                * invoking OOM. Bail if we are suspending
> +                */
> +               if (pm_suspending())
> +                       goto nopage;
>        }
>
>        /* Check if we should retry the allocation */
>
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