linux-kernel - Re: [PATCH v2 2/7] mm/swapfile.c: Replace some #ifdef with IS

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Date:   Wed, 18 Jul 2018 11:25:56 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shaohua Li <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 2/7] mm/swapfile.c: Replace some #ifdef with IS_ENABLED()

Dave Hansen <dave.hansen@...ux.intel.com> writes:

>> @@ -878,6 +877,11 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
>>  	unsigned long offset, i;
>>  	unsigned char *map;
>>  
>> +	if (!IS_ENABLED(CONFIG_THP_SWAP)) {
>> +		VM_WARN_ON_ONCE(1);
>> +		return 0;
>> +	}
>
> I see you seized the opportunity to keep this code gloriously
> unencumbered by pesky comments.  This seems like a time when you might
> have slipped up and been temped to add a comment or two.  Guess not. :)
>
> Seriously, though, does it hurt us to add a comment or two to say
> something like:
>
> 	/*
> 	 * Should not even be attempting cluster allocations when
> 	 * huge page swap is disabled.  Warn and fail the allocation.
> 	 */
> 	if (!IS_ENABLED(CONFIG_THP_SWAP)) {
> 		VM_WARN_ON_ONCE(1);
> 		return 0;
> 	}

I totally agree with you that we should add more comments for THP swap
to improve the code readability.  As for this specific case,
VM_WARN_ON_ONCE() here is just to capture some programming error during
development.  Do we really need comments here?

I will try to add more comments for other places in code regardless this
one.

Best Regards,
Huang, Ying