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SubjectRe: [patch 6/7 -mm] oom: avoid oom killer for lowmem allocations
On Fri, 12 Feb 2010, KAMEZAWA Hiroyuki wrote:

> From viewpoint of panic-on-oom lover, this patch seems to cause regression.
> please do this check after sysctl_panic_on_oom == 2 test.
> I think it's easy. So, temporary Nack to this patch itself.
> And I think calling notifier is not very bad in the situation.
> ==
> void out_of_memory()
> ..snip..
> blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
> So,
> if (sysctl_panic_on_oom == 2) {
> dump_header(NULL, gfp_mask, order, NULL);
> panic("out of memory. Compulsory panic_on_oom is selected.\n");
> }
> if (gfp_zone(gfp_mask) < ZONE_NORMAL) /* oom-kill is useless if lowmem is exhausted. */
> return;
> is better. I think.

I can't agree with that assessment, I don't think it's a desired result to
ever panic the machine regardless of what /proc/sys/vm/panic_on_oom is set
to because a lowmem page allocation fails especially considering, as
mentioned in the changelog, these allocations are never __GFP_NOFAIL and
returning NULL is acceptable.

I've always disliked panicking the machine when a cpuset or mempolicy
allocation fails and panic_on_oom is set to 2. Since both such
constraints now force an iteration of the tasklist when oom_kill_quick is
not enabled and we strictly prohibit the consideration of tasks with
disjoint cpuset mems or mempolicy nodes, I think I'll take this
opportunity to get rid of the panic_on_oom == 2 behavior and ask that
users who really do want to panic the entire machine for cpuset or
mempolicy constrained ooms to simply set all such tasks to OOM_DISABLE.

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