Re: [PATCH] mm-add-vfree_atomic-fix
From: Andy Lutomirski
Date: Tue Dec 13 2016 - 22:02:49 EST
On Tue, Dec 13, 2016 at 11:21 AM, Andrey Ryabinin
> On 12/13/2016 09:15 PM, Andy Lutomirski wrote:
>>>> But not quite acked by me. What happened to the vfree code that
>>>> causes vfree_deferred to be called in a preemptable context? That
>>>> sounds like a bug.
>>> Not sure I understand but the above stack points to a preemptible
>>> context (copy_process). My stack was different and it looks preemptible as well.
>>> free_thread_stack calls vfree_atomic unconditionally. So I am not sure
>>> why do you think this is a bug?
>>>> (This code doesn't exist in Linus' tree. What tree does this apply to.)
>>> Anyway, now that I am looking at Andrew's tree I can see  which
>>> doesn't have this_cpu_ptr. So I am not sure where this this_cpu_ptr came
>>> from. Maybe the previous version of the patch which has shown up in the
>>> linux-next and Andrew has picked up  in the meantime. /me confused
>>>  http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-add-vfree_atomic.patch
>>>  http://lkml.kernel.org/r/1481553981-3856-1-git-send-email-aryabinin@xxxxxxxxxxxxx
>> The underlying issue seems to be that we have this shiny new function
>> vfree_atomic() which doesn't work in *non-atomic* context and that we
> It does work non-atomic context. It's fixed now.
>> have "kernel/fork: use vfree_atomic() to free thread stack" that calls
>> vfree_atomic() from non-atomic context.
> From both context actually. Usually task stack is freed from atomic context:
> On rare occasions it can be freed from non-atomic context, e.g. error path in copy_process().
>> I'm not sure what the motivation of the latter patch was, but ISTM we
>> should revert it. TBH I'm not quite sure what the purpose of
>> splitting vfree() and vfree_atomic() was, but I'm not seeing any
>> reason that the common case of freeing stacks from non-atomic context
>> should defer the free instead of just doing it right away.
>> Andrey, Johannes, why should task stack freeing use vfree_atomic() in
>> the first place?
> Because vfree() now can sleep and task stack freeing usually done in atomic context.
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