linux-kernel - Re: [PATCH v1] mm/mempolicy: fix migrate_to_node() assuming there is at least one VMA in a MM

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Message-ID: <7e6c2c57-d633-4052-95a9-31ccc9c66327@redhat.com>
Date: Fri, 22 Nov 2024 15:57:58 +0100
From: David Hildenbrand <david@...hat.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, syzbot+3511625422f7aa637f0d@...kaller.appspotmail.com,
 stable@...r.kernel.org, Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH v1] mm/mempolicy: fix migrate_to_node() assuming there is
 at least one VMA in a MM

On 22.11.24 15:44, Liam R. Howlett wrote:
> * David Hildenbrand <david@...hat.com> [241122 04:32]:
>> On 22.11.24 07:19, Andrew Morton wrote:
>>> On Wed, 20 Nov 2024 15:27:46 -0500 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
>>>
>>>> I hate the extra check because syzbot can cause this as this should
>>>> basically never happen in real life, but it seems we have to add it.
>>>
>>> So..
>>>
>>> --- a/mm/mempolicy.c~mm-mempolicy-fix-migrate_to_node-assuming-there-is-at-least-one-vma-in-a-mm-fix
>>> +++ a/mm/mempolicy.c
>>> @@ -1080,7 +1080,7 @@ static long migrate_to_node(struct mm_st
>>>    	mmap_read_lock(mm);
>>>    	vma = find_vma(mm, 0);
>>> -	if (!vma) {
>>> +	if (unlikely(!vma)) {
>>>    		mmap_read_unlock(mm);
>>>    		return 0;
>>>    	}
>>> _
>>>
>>> ?
>>
>> Why not, at least for documentation purposes. Because I don't think this is
>> any fast-path we really care about, so expect the runtime effects to be
>> mostly negligible. Thanks!
> 
> The next email we get about this will be a bot with a micro benchmark
> performance drop.

I'll be pleasantly surprised if our migration code is that fast such 
that this here would matter :)

-- 
Cheers,

David / dhildenb