Re: [PATCH RFC v2 2/2] mm: Rework swap handling of zap_pte_range

On 11/16/21 05:11, Matthew Wilcox wrote:

On Tue, Nov 16, 2021 at 12:51:13AM -0800, John Hubbard wrote:

quoted

On 11/15/21 05:57, Matthew Wilcox wrote:

quoted

On Mon, Nov 15, 2021 at 09:49:51PM +0800, Peter Xu wrote:

quoted

Clean the code up by merging the device private/exclusive swap entry handling
with the rest, then we merge the pte clear operation too.

struct* page is defined in multiple places in the function, move it upward.

Is that actually a good thing?  There was a time when declaring

Yes. It is a very good thing. Having multiple cases of shadowed variables
(in this case I'm using programming language terminology, or what I
remember it as, anyway) provides lots of opportunities to create
hard-to-spot bugs.

I think you're misremembering.  These are shadowed variables:

OK, I remembered correctly, but read the diffs a little too quickly, and...

int a;

int b(void)
{
	int a;
	if (c) {
		int a;
	}
}

This is not:

int b(void)
{

...missed that there is no longer a "int a" at the top level. But it does
still present a small land mine, in that just adding a top level "int a"
creates all these shadowed variables (not necessarily bugs, yet, I know).

It's less of an issue here, then I first thought. Generally, it's probably best
to either use "int a" throughout, or differently named variables at lower
levels...or make smaller functions. Because if a variable name is reused
a lot in the same function then there is likely a relationship of sorts
between the instances, and it's worth deciding what that is.

	if (c) {
		int a;
	} else {
		int a;
	}
}

I really wish we could turn on -Wshadow, but we get compilation warnings
from header files right now.  Or we did last time I checked.

...and as you say, it would be nice if the programmer could just let the
compiler figure out if there is a real problem. The elaborate rituals to
stay out of harm's way are not as good as a tool that checks. :)

thanks,
-- 
John Hubbard
NVIDIA