On Tue, 2019-10-01 at 12:04 -0700, John Hubbard wrote:

On 10/1/19 10:56 AM, Leonardo Bras wrote:

quoted

On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:

quoted

On 9/27/19 4:40 PM, Leonardo Bras wrote:

...

quoted

quoted

quoted

diff --git a/mm/gup.c b/mm/gup.c
index 98f13ab37bac..7105c829cf44 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
 {
+	struct mm_struct *mm;

I don't think that this local variable adds any value, so let's not use it.
Similar point in a few other patches too.

It avoids 1 deference of current->mm, it's a little performance gain.

No, it isn't. :) 

Longer answer: at this level (by which I mean, "wrote the C code, haven't looked
at the generated asm yet, and haven't done a direct perf test yet"), none of us
C programmers are entitled to imagine that we can second guess both the compiler 
and the CPU well enough to claim that  declaring a local pointer variable on the
stack will even *affect* performance, much less know which way it will go!

I did this based on how costly can be 'current', and I could notice
reduction in assembly size most of the time. (powerpc)
But I get what you mean, maybe the (possible) performance gain don't
worth the extra work.

The compiler at -O2 will *absolutely* optimize away any local variables that
it doesn't need.

And that leads to how kernel programmers routinely decide about that kind of 
variable: "does the variable's added clarity compensate for the extra visual 
noise and for the need to manage the variable?"

That's a good way to decide it. :)

Here, and in most (all?) other points in the patchset where you've added an
mm local variable, the answer is no.

Well, IMHO it's cleaner that way. But I get that other people may
disagree. 

...	start_lockless_pgtbl_walk(mm);

quoted

quoted

Minor: I'd like to rename this register_lockless_pgtable_walker().

quoted

 		local_irq_disable();
 		gup_pgd_range(addr, end, gup_flags, pages, &nr);
 		local_irq_enable();
+		end_lockless_pgtbl_walk(mm);

...and deregister_lockless_pgtable_walker().

I have no problem changing the name, but I don't register/deregister
are good terms for this. 

I would rather use start/finish, begin/end, and so on. Register sounds
like something more complicated than what we are trying to achieve
here. 

OK, well, I don't want to bikeshed on naming more than I usually do, and 
what you have is reasonable, so I'll leave that alone. :)

thanks,

Thank for the feedback,