> It is possible to remove the special pagefault oom handler by simply oom
> locking all system zones and then calling directly into out_of_memory().
>
> All populated zones must have ZONE_OOM_LOCKED set, otherwise there is a
> parallel oom killing in progress that will lead to eventual memory freeing
> so it's not necessary to needlessly kill another task. The context in
> which the pagefault is allocating memory is unknown to the oom killer, so
> this is done on a system-wide level.
>
> If a task has already been oom killed and hasn't fully exited yet, this
> will be a no-op since select_bad_process() recognizes tasks across the
> system with TIF_MEMDIE set.
>
> Acked-by: Nick Piggin <npiggin@xxxxxxx>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/oom_kill.c | 86 +++++++++++++++++++++++++++++++++++++-------------------
> 1 files changed, 57 insertions(+), 29 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -615,6 +615,44 @@ void clear_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
> }
>
> /*
> + * Try to acquire the oom killer lock for all system zones. Returns zero if a
> + * parallel oom killing is taking place, otherwise locks all zones and returns
> + * non-zero.
> + */
> +static int try_set_system_oom(void)
> +{
> + struct zone *zone;
> + int ret = 1;
> +
> + spin_lock(&zone_scan_lock);
> + for_each_populated_zone(zone)
> + if (zone_is_oom_locked(zone)) {
> + ret = 0;
> + goto out;
> + }
> + for_each_populated_zone(zone)
> + zone_set_flag(zone, ZONE_OOM_LOCKED);
> +out:
> + spin_unlock(&zone_scan_lock);
> + return ret;
> +}
> +
> +/*
> + * Clears ZONE_OOM_LOCKED for all system zones so that failed allocation
> + * attempts or page faults may now recall the oom killer, if necessary.
> + */
> +static void clear_system_oom(void)
> +{
> + struct zone *zone;
> +
> + spin_lock(&zone_scan_lock);
> + for_each_populated_zone(zone)
> + zone_clear_flag(zone, ZONE_OOM_LOCKED);
> + spin_unlock(&zone_scan_lock);
> +}
> +
> +
> +/*
> * Must be called with tasklist_lock held for read.
> */
> static void __out_of_memory(gfp_t gfp_mask, int order,
> @@ -649,33 +687,6 @@ retry:
> goto retry;
> }
>
> -/*
> - * pagefault handler calls into here because it is out of memory but
> - * doesn't know exactly how or why.
> - */
> -void pagefault_out_of_memory(void)
> -{
> - unsigned long freed = 0;
> -
> - blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
> - if (freed > 0)
> - /* Got some memory back in the last second. */
> - return;
> -
> - check_panic_on_oom(CONSTRAINT_NONE, 0, 0);
> - read_lock(&tasklist_lock);
> - /* unknown gfp_mask and order */
> - __out_of_memory(0, 0, CONSTRAINT_NONE, NULL);
> - read_unlock(&tasklist_lock);
> -
> - /*
> - * Give "p" a good chance of killing itself before we
> - * retry to allocate memory.
> - */
> - if (!test_thread_flag(TIF_MEMDIE))
> - schedule_timeout_uninterruptible(1);
> -}
> -
> /**
> * out_of_memory - kill the "best" process when we run out of memory
> * @zonelist: zonelist pointer
> @@ -692,7 +703,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
> int order, nodemask_t *nodemask)
> {
> unsigned long freed = 0;
> - enum oom_constraint constraint;
> + enum oom_constraint constraint = CONSTRAINT_NONE;
>
> blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
> if (freed > 0)
> @@ -713,7 +724,8 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
> * Check if there were limitations on the allocation (only relevant for
> * NUMA) that may require different handling.
> */
> - constraint = constrained_alloc(zonelist, gfp_mask, nodemask);
> + if (zonelist)
> + constraint = constrained_alloc(zonelist, gfp_mask, nodemask);
> check_panic_on_oom(constraint, gfp_mask, order);
> read_lock(&tasklist_lock);
> __out_of_memory(gfp_mask, order, constraint, nodemask);
> @@ -726,3 +738,19 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
> if (!test_thread_flag(TIF_MEMDIE))
> schedule_timeout_uninterruptible(1);
> }
> +
> +/*
> + * The pagefault handler calls here because it is out of memory, so kill a
> + * memory-hogging task. If a populated zone has ZONE_OOM_LOCKED set, a parallel
> + * oom killing is already in progress so do nothing. If a task is found with
> + * TIF_MEMDIE set, it has been killed so do nothing and allow it to exit.
> + */
> +void pagefault_out_of_memory(void)
> +{
> + if (try_set_system_oom()) {
> + out_of_memory(NULL, 0, 0, NULL);
> + clear_system_oom();
> + }
> + if (!test_thread_flag(TIF_MEMDIE))
> + schedule_timeout_uninterruptible(1);
> +}
this one is already there in my patch kit.
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