Re: [PATCH v13 1/6] mm/vmstat: Add CPU-specific variable to track a vmstat discrepancy

[Frederic Weisbecker]

On Tue, Jan 10, 2023 at 12:58:32PM +0100, Christoph Lameter wrote:
> On Thu, 5 Jan 2023, Marcelo Tosatti wrote:
> 
> > +static inline void vmstat_mark_dirty(void)
> > +{
> > +	this_cpu_write(vmstat_dirty, true);
> > +}
> 
> this_cpu_write() is intended for an per cpu atomic context. You are not
> using it in that way. The processor may have changed before or after and
> thus vmstat_dirty for another CPU may  have been marked dirty.
> 
> I guess this would have to be called __vmstat_mark_dirty() and be using
> __this_cpu_write(*) with a requirement that preemption be disabled before
> using this function.

You're right. So this patchset also arranges for these vmstat functions to be
called with preemption disabled. I'm converting the this_cpu operations
to __this_cpu versions to make sure of that. And I believe the __this_cpu
WARN if preemptible().


> 
> > +static inline void vmstat_clear_dirty(void)
> > +{
> > +	this_cpu_write(vmstat_dirty, false);
> > +}
> 
> Same
> 
> > +static inline bool is_vmstat_dirty(void)
> > +{
> > +	return this_cpu_read(vmstat_dirty);
> > +}
> 
> This function would only work correctly if preemption is disabled.
> Otherwise the processor may change.

Indeed that should apply as __this_cpu_read() as well.

Thanks!