Re: [PATCH] mm: do not rely on preempt_count in print_vma_addr

["Yang Shi" <yang.s@alibaba-inc.com>]

On 11/6/17 5:40 AM, Michal Hocko wrote:
> On Mon 06-11-17 13:12:22, Michal Hocko wrote:
>> On Mon 06-11-17 13:00:25, Peter Zijlstra wrote:
>>> On Mon, Nov 06, 2017 at 11:43:54AM +0100, Michal Hocko wrote:
>>>>> Yes the comment is very much accurate.
>>>>
>>>> Which suggests that print_vma_addr might be problematic, right?
>>>> Shouldn't we do trylock on mmap_sem instead?
>>>
>>> Yes that's complete rubbish. trylock will get spurious failures to print
>>> when the lock is contended.
>>
>> Yes, but I guess that it is acceptable to to not print the state under
>> that condition.
> 
> So what do you think about this? I think this is more robust than
> playing tricks with the explicit preempt count checks and less tedious
> than checking to make it conditional on the context. This is on top of
> Linus tree and if accepted it should replace the patch discussed here.
> ---
>  From 0de6d57cbc54ee2686d1f1e4ffcc4ed490ded8aa Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Mon, 6 Nov 2017 14:31:20 +0100
> Subject: [PATCH] mm: do not rely on preempt_count in print_vma_addr
> 
> The preempt count check on print_vma_addr has been added by e8bff74afbdb
> ("x86: fix "BUG: sleeping function called from invalid context" in
> print_vma_addr()") and it relied on the elevated preempt count from
> preempt_conditional_sti because preempt_count check doesn't work on
> non preemptive kernels by default. The code has evolved though and
> d99e1bd175f4 ("x86/entry/traps: Refactor preemption and interrupt flag
> handling") has replaced preempt_conditional_sti by an explicit
> preempt_disable which is noop on !PREEMPT so the check in print_vma_addr
> is broken.
> 
> Fix the issue by using trylock on mmap_sem rather than chacking the

s/chacking/checking

> preempt count. The allocation we are relying on has to be GFP_NOWAIT
> as well. There is a chance that we won't dump the vma state if the lock
> is contended or the memory short but this is acceptable outcome and much
> less fragile than the not working preemption check or tricks around it.
> 
> Fixes: d99e1bd175f4 ("x86/entry/traps: Refactor preemption and interrupt flag handling")
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Acked-by: Yang Shi <yang.s@alibaba-inc.com>

Regards,
Yang

> ---
>   mm/memory.c | 8 +++-----
>   1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index a728bed16c20..1e308ac8ca0a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4457,17 +4457,15 @@ void print_vma_addr(char *prefix, unsigned long ip)
>   	struct vm_area_struct *vma;
>   
>   	/*
> -	 * Do not print if we are in atomic
> -	 * contexts (in exception stacks, etc.):
> +	 * we might be running from an atomic context so we cannot sleep
>   	 */
> -	if (preempt_count())
> +	if (!down_read_trylock(&mm->mmap_sem))
>   		return;
>   
> -	down_read(&mm->mmap_sem);
>   	vma = find_vma(mm, ip);
>   	if (vma && vma->vm_file) {
>   		struct file *f = vma->vm_file;
> -		char *buf = (char *)__get_free_page(GFP_KERNEL);
> +		char *buf = (char *)__get_free_page(GFP_NOWAIT);
>   		if (buf) {
>   			char *p;
>   
>