linux-kernel - Re: [PATCH V2] fork: Improve error message for corrupted page tables

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Date:   Tue, 06 Aug 2019 11:30:02 -0700
From:   Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Ingo Molnar <mingo@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH V2] fork: Improve error message for corrupted page tables

On Tue, 2019-08-06 at 09:30 -0700, Dave Hansen wrote:
> On 8/5/19 8:05 PM, Sai Praneeth Prakhya wrote:
> > +static const char * const resident_page_types[NR_MM_COUNTERS] = {
> > +	[MM_FILEPAGES]		= "MM_FILEPAGES",
> > +	[MM_ANONPAGES]		= "MM_ANONPAGES",
> > +	[MM_SWAPENTS]		= "MM_SWAPENTS",
> > +	[MM_SHMEMPAGES]		= "MM_SHMEMPAGES",
> > +};
> 
> One trick to ensure that this gets updated if the names are ever
> updated.  You can do:
> 
> #define NAMED_ARRAY_INDEX(x)	[x] = __stringify(x),
> 
> and
> 
> static const char * const resident_page_types[NR_MM_COUNTERS] = {
> 	NAMED_ARRAY_INDEX(MM_FILE_PAGES),
> 	NAMED_ARRAY_INDEX(MM_SHMEMPAGES),
> 	...
> };

Thanks for the suggestion Dave. I will add this in V3.
Even with this, (if ever) anyone who changes the name of page types or adds an
new entry would still need to update struct resident_page_types[]. So, I will
add the comment as suggested by Vlastimil.

> 
> That makes sure that any name changes make it into the strings.  Then
> stick a:
> 
> 	BUILD_BUG_ON(NR_MM_COUNTERS != ARRAY_SIZE(resident_page_types));
> 
> somewhere.  That makes sure that any new array indexes get a string
> added in the array.  Otherwise you get nice, early, compile-time errors.

Sure! this sounds good and a small nit-bit :)
For the BUILD_BUG_ON() to work, the definition of struct should be changed as
below

static const char * const resident_page_types[] = {
...
}

i.e. we should not specify the size of array.

Regards,
Sai