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

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Message-ID: <YZOt7qD6yeSXJgv4@casper.infradead.org>
Date:   Tue, 16 Nov 2021 13:11:10 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
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 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:

int a;

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

This is not:

int b(void)
{
	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.