linux-kernel - Re: [PATCH 4/7] mm: defer vmalloc from atomic context

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Date:   Mon, 7 Nov 2016 18:01:45 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Joel Fernandes <joelaf@...gle.com>
CC:     Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jisheng Zhang <jszhang@...vell.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        John Dias <joaodias@...gle.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 4/7] mm: defer vmalloc from atomic context

On 11/05/2016 06:43 AM, Joel Fernandes wrote:
> On Mon, Oct 24, 2016 at 8:44 AM, Andrey Ryabinin
> <aryabinin@...tuozzo.com> wrote:
>>
>>
>> On 10/22/2016 06:17 PM, Christoph Hellwig wrote:
>>> We want to be able to use a sleeping lock for freeing vmap to keep
>>> latency down.  For this we need to use the deferred vfree mechanisms
>>> no only from interrupt, but from any atomic context.
>>>
>>> Signed-off-by: Christoph Hellwig <hch@....de>
>>> ---
>>>  mm/vmalloc.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index a4e2cec..bcc1a64 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -1509,7 +1509,7 @@ void vfree(const void *addr)
>>>
>>>       if (!addr)
>>>               return;
>>> -     if (unlikely(in_interrupt())) {
>>> +     if (unlikely(in_atomic())) {
>>
>> in_atomic() cannot always detect atomic context, thus it shouldn't be used here.
>> You can add something like vfree_in_atomic() and use it in atomic call sites.
>>
> 
> So because in_atomic doesn't work for !CONFIG_PREEMPT kernels, can we
> always defer the work in these cases?
> 
> So for non-preemptible kernels, we always defer:
> 
> if (!IS_ENABLED(CONFIG_PREEMPT) || in_atomic()) {
>   // defer
> }
> 
> Is this fine? Or any other ideas?
> 

What's wrong with my idea?
We can add vfree_in_atomic() and use it to free vmapped stacks
and for any other places where vfree() used 'in_atomict() && !in_interrupt()' context.