linux-kernel - Re: [PATCH RFC v2 2/2] mm: Rework swap handling of zap_pte

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Message-ID: <b055cf62-c957-db35-2bf9-630345d63cf6@nvidia.com>
Date:   Tue, 16 Nov 2021 11:06:18 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: 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:
>> On 11/15/21 05:57, Matthew Wilcox wrote:
>>> On Mon, Nov 15, 2021 at 09:49:51PM +0800, Peter Xu wrote:
>>>> 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