linux-kernel - Re: [PATCH] mm: use in_atomic() in print_vma

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Date:   Fri, 3 Nov 2017 09:29:00 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.s@...baba-inc.com>
Cc:     mingo@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use in_atomic() in print_vma_addr()

On Fri 03-11-17 01:44:44, Yang Shi wrote:
> 
> 
> On 11/2/17 12:57 AM, Michal Hocko wrote:
> > On Thu 02-11-17 05:38:33, Yang Shi wrote:
> > > commit 3e51f3c4004c9b01f66da03214a3e206f5ed627b
> > > ("sched/preempt: Remove PREEMPT_ACTIVE unmasking off in_atomic()") makes
> > > in_atomic() just check the preempt count, so it is not necessary to use
> > > preempt_count() in print_vma_addr() any more. Replace preempt_count() to
> > > in_atomic() which is a generic API for checking atomic context.
> > 
> > But why? Is there some general work to get rid of the direct preempt_count
> > usage outside of the generic API?
> 
> I may not articulate it in the commit log, I would say "in_atomic" is
> *preferred* API for checking atomic context instead of preempt_count() which
> should be used for retrieving the preemption count value.
> 
> I would say there is not such general elimination work undergoing right now,
> but if we go through the kernel code, almost everywhere "in_atomic" is used
> for such use case already, except two places:
> 
> - print_vma_addr()
> - debug_smp_processor_id()
> 
> Both came from Ingo long time ago before commit
> 3e51f3c4004c9b01f66da03214a3e206f5ed627b ("sched/preempt: Remove
> PREEMPT_ACTIVE unmasking off in_atomic()"). But, after this commit was
> merged, I don't see why *not* use in_atomic() to follow the convention.

OK.

Acked-by: Michal Hocko <mhocko@...e.com>
> 
> Thanks,
> Yang
> 
> > 
> > > Signed-off-by: Yang Shi <yang.s@...baba-inc.com>
> > > ---
> > >   mm/memory.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index a728bed..19b684e 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -4460,7 +4460,7 @@ void print_vma_addr(char *prefix, unsigned long ip)
> > >   	 * Do not print if we are in atomic
> > >   	 * contexts (in exception stacks, etc.):
> > >   	 */
> > > -	if (preempt_count())
> > > +	if (in_atomic())
> > >   		return;
> > >   	down_read(&mm->mmap_sem);
> > > -- 
> > > 1.8.3.1
> > 
> 
> --
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-- 
Michal Hocko
SUSE Labs