Re: [PATCH v4] mm/page_owner: clamp read count to PAGE_SIZE

[Matthew Wilcox]

On Thu, Nov 01, 2018 at 04:30:12PM -0700, Joe Perches wrote:
> On Thu, 2018-11-01 at 14:47 -0700, Andrew Morton wrote:
> > +++ a/mm/page_owner.c
> > @@ -351,7 +351,7 @@ print_page_owner(char __user *buf, size_
> >  		.skip = 0
> >  	};
> >  
> > -	count = count > PAGE_SIZE ? PAGE_SIZE : count;
> > +	count = min_t(size_t, count, PAGE_SIZE);
> >  	kbuf = kmalloc(count, GFP_KERNEL);
> >  	if (!kbuf)
> >  		return -ENOMEM;
> 
> A bit tidier still might be
> 
> 	if (count > PAGE_SIZE)
> 		count = PAGE_SIZE;
> 
> as that would not always cause a write back to count.

90% chance 'count' is already in a register and will stay there.  99.9%
chance that if it's not in a register, it's on the top of the stack,
which is by definition a hot, local, dirty cacheline.

What you're saying makes sense for a struct which might well be in a
shared cacheline state.  But for a function-local variable?  No.