Re: [PATCH 02/11] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code

Oleg Nesterov
2015-09-23T11:47:59Z

On 09/22, Linus Torvalds wrote:
>
> However, this now becomes a pattern for the series, and that just makes me think
>
> "Why is this not a 'for_each_mm()' pattern helper?"

And we already have other users. And note that oom_kill_process() does _not_
follow this pattern and that is why it is buggy.

So this is funny, but I was thinking about almost the same, something like

struct task_struct *next_task_with_mm(struct task_struct *p)
{
struct task_struct *t;

p = p->group_leader;
while ((p = next_task(p)) != &init_task) {
if (p->flags & PF_KTHREAD)
continue;

t = find_lock_task_mm(p);
if (t)
return t;
}

return NULL;
}

#define for_each_task_lock_mm(p)
for (p = &init_task; (p = next_task_with_mm(p)); task_unlock(p))

So that you can do

for_each_task_lock_mm(p) {
do_something_with(p->mm);

if (some_condition()) {
// UNFORTUNATELY you can't just do "break"
task_unlock(p);
break;
}
}

do you think it makes sense?

In fact it can't be simpler, we can move task_unlock() into next_task_with_mm(),
it can check ->mm != NULL or p != init_task.

Oleg.