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SubjectRe: [PATCH 2/2] mm: fix mm_struct reference counting bugs in mm/oom_kill.c
On Friday 14 April 2006 00:26, Andrew Morton wrote:
> task_lock() can be used to pin a task's ->mm. To use task_lock() in
> badness() we'd need to either
> a) nest task_lock()s. I don't know if we're doing that anywhere else,
> but the parent->child ordering is a natural one. or
> b) take a ref on the parent's mm_struct, drop the parent's task_lock()
> while we walk the children, then do mmput() on the parent's mm outside
> tasklist_lock. This is probably better.

Looking a bit more closely at the code, I see that
select_bad_process() iterates over all tasks, repeatedly calling
badness(). This would complicate option 'b' since the iteration is
done while holding tasklist_lock. An alternative to option 'a' that
avoids nesting task_lock()s would be to define a couple of new
functions that might look something like this:

void mmput_atomic(struct mm_struct *mm)
if (atomic_dec_and_test(&mm->mm_users)) {
add mm to a global list of expired mm_structs

void mmput_atomic_cleanup(void)
empty the global list of expired mm_structs and do
cleanup stuff for each one

Then you could call mmput_atomic() an arbitrary # of times in places
where sleeping is not permitted, as long as mmput_atomic_cleanup() is
later called in a place where sleeping is permissible. In the case
of the OOM killer code, a call to mmput_atomic_cleanup() could be
added to out_of_memory() in a place where we no longer hold
tasklist_lock. Let me know if you have a preference for either of
these options, or if you have other suggestions.

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