linux-kernel - Re: [PATCH] mm: centralize and fix max map count limit checking

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Message-ID: <84aad392-3bff-4f98-b612-5e9a046edb36@redhat.com>
Date: Thu, 4 Sep 2025 19:41:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, akpm@...ux-foundation.org,
 minchan@...nel.org, kernel-team@...roid.com, android-mm@...gle.com,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: centralize and fix max map count limit checking

On 04.09.25 19:33, Lorenzo Stoakes wrote:
> On Thu, Sep 04, 2025 at 01:22:51PM -0400, Liam R. Howlett wrote:
>>>> diff --git a/mm/mremap.c b/mm/mremap.c
>>>> index e618a706aff5..793fad58302c 100644
>>>> --- a/mm/mremap.c
>>>> +++ b/mm/mremap.c
>>>> @@ -1040,7 +1040,7 @@ static unsigned long prep_move_vma(struct vma_remap_struct *vrm)
>>>>   	 * We'd prefer to avoid failure later on in do_munmap:
>>>>   	 * which may split one vma into three before unmapping.
>>>>   	 */
>>>> -	if (current->mm->map_count >= sysctl_max_map_count - 3)
>>>> +	if (exceeds_max_map_count(current->mm, 4))
>>>>   		return -ENOMEM;
>>>
>>> In my version this would be:
>>>
>>> 	if (map_count_capacity(current->mm) < 4)
>>> 		return -ENOMEM;
>>>
>>
>> Someone could write map_count_capacity(current->mm) <= 4 and reintroduce
>> what this is trying to solve.  And with the way it is written in this
>> patch, someone could pass in the wrong number.
> 
> Right, but I think 'capacity' is pretty clear here, if the caller does something
> silly then that's on them...
> 
>>
>> I'm not sure this is worth doing.  There are places we allow the count
>> to go higher.
> 
> ...But yeah, it's kinda borderline as to how useful this is.
> 
> I _do_ however like the 'put map count in one place statically' rather than
> having a global, so a minimal version of this could be to just have a helper
> function that gets the sysctl_max_map_count, e.g.:
> 
> if (current->mm->mmap_count >= max_map_count() - 3)

I enjoy seeing sysctl_max_map_count hidden. But map_count_capacity() is 
even more readable, so I like it.

I don't complete like the "capacity" term, but I cannot think of 
something better right now. Maybe something around "free" or 
"remaining", not sure.

I also don't completely like "map_count" (I know, I know, we call it 
like that in structures), because it reminds me of the mapcount ... 
talking somehow about "vmas" would be quite clear.

Anyhow, just as an inspiration my 2 cents ...

-- 
Cheers

David / dhildenb