Message-ID: <6e2f8421-b8b7-c8ec-7a2a-646efb134bbe@nvidia.com>
Date:   Wed, 24 Nov 2021 21:29:11 -0800
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Subject: Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG
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To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, cgel.zte@gmail.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        chiminghao <chi.minghao@zte.com.cn>,
        Zeal Robot <zealci@zte.com.cn>
References: <20211124030849.34998-1-chi.minghao@zte.com.cn>
 <YZ483gwnwTysPt0G@casper.infradead.org>
 <20211124142751.e48cdcc3aea9e0ef899f4347@linux-foundation.org>
 <348f3a1f-c7d3-c21f-419d-7acd2e5290b6@nvidia.com>
 <YZ8Dz/85n1Dogy87@casper.infradead.org>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <YZ8Dz/85n1Dogy87@casper.infradead.org>
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On 11/24/21 19:32, Matthew Wilcox wrote:
> On Wed, Nov 24, 2021 at 02:45:59PM -0800, John Hubbard wrote:
>> @@ -2201,13 +2201,12 @@ static int __ref try_remove_memory(u64 start, u64 size)
>> */
>> void __remove_memory(u64 start, u64 size)
>> {
>> -
>> +       int ret = try_remove_memory(start, size);
>> /*
>> * trigger BUG() if some memory is not offlined prior to calling this
>> * function
>> */
>> -       if (try_remove_memory(start, size))
>> -               BUG();
>> +       BUG_ON(ret);
>> }
> 
> I'd rather leave it the way it is.  I don't see why the version you
> propose is better.


In isolation, it's *not* better. It's only potentially useful in the
context of "code plus tools". That is to say, if the coccinelle change
request were rejected, then this provides a way forward that is not
worse than the existing code, and also works around the warning.


> 
>> ...and by the way, while going to type that, I immediately stumbled upon
>> another pre-existing case of this sort of thing, in try_remove_memory(),
>> which does this:
>>
>> static int __ref try_remove_memory(u64 start, u64 size)
>> {
>> 	struct vmem_altmap mhp_altmap = {};
>> 	struct vmem_altmap *altmap = NULL;
>> 	unsigned long nr_vmemmap_pages;
>> 	int rc = 0, nid = NUMA_NO_NODE;
>>
>> 	BUG_ON(check_hotplug_memory_range(start, size));
> 
> That needs to be fixed.


Yes it does. :) I pointed it out in hopes that Chiminghao might be inspired
to go find and fix some of these.


thanks,
-- 
John Hubbard
NVIDIA