Re: [PATCH] ksm: hold anon_vma in rmap_item fix

[Balbir Singh]

* Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx> [2009-11-29 15:50:32]:

> KSM mem_cgroup testing oopsed on NULL pointer in mem_cgroup_from_task(),
> called from the mm_match_cgroup() in page_referenced_ksm().
> 
> Right, it is inappropriate to use mm_match_cgroup() on rmap_item->mm
> there: that mm could be waiting for ksmd's final mmdrop(), with its
> mm->owner task long gone.
> 
> Move the mm_match_cgroup() test down into the anon_vma loop, which is
> where it now should be to match page_referenced_anon().  The anon_vma
> guarantees its vmas are valid, which guarantee their mms are valid.
> 
> However... although this moves the oops from easy-to-reproduce to
> never-seen, I think we shall want to do more later: so far as I can
> see, with or without KSM, the use of mm->owner from page_referenced()
> is unsafe.  No problem when NULL, but it may have been left pointing
> to a task_struct freed by now, with nonsense in mm->owner->cgroups.
>

Ideally we should not be left pointing to a stale task struct, unless
our assumption about mm_users is incorrect (discussed below).

 
> But let's put this patch in while we discuss that separately: perhaps
> mm_need_new_owner() should not short-circuit when mm_users <= 1, or
> perhaps it should then set mm->owner to NULL, or perhaps we abandon
> mm->owner as more trouble than it's worth, or... perhaps I'm wrong.
> 

We short circuit, since the task is exiting and mm_users <= 1 and we
are shorting going to do a mmput(). I suspect what you are seeing is
mm_count >= 1 and mm_users == 0. With users == 0, we should set
owner to NULL

We could look for the above condition in mmput() and clear the owner
when users become 0.


-- 
	Balbir
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/