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

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Message-ID: <8e1881b8-3867-4cea-b03e-50c05ed8148d@redhat.com>
Date: Thu, 4 Sep 2025 21:02:30 +0200
From: David Hildenbrand <david@...hat.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Kalesh Singh <kaleshsingh@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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 20:49, Liam R. Howlett wrote:
> * Kalesh Singh <kaleshsingh@...gle.com> [250904 13:51]:
>> On Thu, Sep 4, 2025 at 10:42 AM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> 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.
>>
>> Thanks David, my original implementation started with vma_limit() :).
>> Maybe something like vma_count_remaining() ?
> 
> Yes, reducing this confusion would very much be helpful.  In fact, if
> you put it in its own function we could change the actual name with
> lower impact.  map_count vs mapcount is annoying.
> 
> vma_headroom() ?
> additional_vma_space() ?

VMA space might be interpreted as VA space.

I think basing it on "vma_count" would be good.

vma_count_capacity()

vma_count_headroom()

vma_count_remaining()

vma_count_avail()

vma_count_left()

> 
> Maybe David would like:
> remedy_vma_space() which would be !poison_vma_space().. that's pretty
> clear.. :)

:D Careful, sensitive topic; I know that Lorenzo is still crying about 
this late at night when nobody watches, screaming "DAVID, WHYYYYYYY" ;)

-- 
Cheers

David / dhildenb